Source file src/go/parser/parser.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package parser implements a parser for Go source files.
     6  //
     7  // The [ParseFile] function reads file input from a string, []byte, or
     8  // io.Reader, and produces an [ast.File] representing the complete
     9  // abstract syntax tree of the file.
    10  //
    11  // The [ParseExprFrom] function reads a single source-level expression and
    12  // produces an [ast.Expr], the syntax tree of the expression.
    13  //
    14  // The parser accepts a larger language than is syntactically permitted by
    15  // the Go spec, for simplicity, and for improved robustness in the presence
    16  // of syntax errors. For instance, in method declarations, the receiver is
    17  // treated like an ordinary parameter list and thus may contain multiple
    18  // entries where the spec permits exactly one. Consequently, the corresponding
    19  // field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
    20  //
    21  // Applications that need to parse one or more complete packages of Go
    22  // source code may find it more convenient not to interact directly
    23  // with the parser but instead to use the Load function in package
    24  // [golang.org/x/tools/go/packages].
    25  package parser
    26  
    27  import (
    28  	"fmt"
    29  	"go/ast"
    30  	"go/build/constraint"
    31  	"go/internal/scannerhooks"
    32  	"go/scanner"
    33  	"go/token"
    34  	"strings"
    35  )
    36  
    37  // The parser structure holds the parser's internal state.
    38  type parser struct {
    39  	file    *token.File
    40  	errors  scanner.ErrorList
    41  	scanner scanner.Scanner
    42  
    43  	// Tracing/debugging
    44  	mode   Mode // parsing mode
    45  	trace  bool // == (mode&Trace != 0)
    46  	indent int  // indentation used for tracing output
    47  
    48  	// Comments
    49  	comments    []*ast.CommentGroup
    50  	leadComment *ast.CommentGroup // last lead comment
    51  	lineComment *ast.CommentGroup // last line comment
    52  	top         bool              // in top of file (before package clause)
    53  	goVersion   string            // minimum Go version found in //go:build comment
    54  
    55  	// Next token
    56  	pos       token.Pos   // token position
    57  	tok       token.Token // one token look-ahead
    58  	lit       string      // token literal
    59  	stringEnd token.Pos   // position immediately after token; STRING only
    60  
    61  	// Error recovery
    62  	// (used to limit the number of calls to parser.advance
    63  	// w/o making scanning progress - avoids potential endless
    64  	// loops across multiple parser functions during error recovery)
    65  	syncPos token.Pos // last synchronization position
    66  	syncCnt int       // number of parser.advance calls without progress
    67  
    68  	// Non-syntactic parser control
    69  	exprLev int  // < 0: in control clause, >= 0: in expression
    70  	inRhs   bool // if set, the parser is parsing a rhs expression
    71  
    72  	imports []*ast.ImportSpec // list of imports
    73  
    74  	// nestLev is used to track and limit the recursion depth
    75  	// during parsing.
    76  	nestLev int
    77  }
    78  
    79  func (p *parser) init(file *token.File, src []byte, mode Mode) {
    80  	p.file = file
    81  	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
    82  	p.scanner.Init(p.file, src, eh, scanner.ScanComments)
    83  
    84  	p.top = true
    85  	p.mode = mode
    86  	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
    87  	p.next()
    88  }
    89  
    90  // ----------------------------------------------------------------------------
    91  // Parsing support
    92  
    93  func (p *parser) printTrace(a ...any) {
    94  	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
    95  	const n = len(dots)
    96  	pos := p.file.Position(p.pos)
    97  	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
    98  	i := 2 * p.indent
    99  	for i > n {
   100  		fmt.Print(dots)
   101  		i -= n
   102  	}
   103  	// i <= n
   104  	fmt.Print(dots[0:i])
   105  	fmt.Println(a...)
   106  }
   107  
   108  func trace(p *parser, msg string) *parser {
   109  	p.printTrace(msg, "(")
   110  	p.indent++
   111  	return p
   112  }
   113  
   114  // Usage pattern: defer un(trace(p, "..."))
   115  func un(p *parser) {
   116  	p.indent--
   117  	p.printTrace(")")
   118  }
   119  
   120  // maxNestLev is the deepest we're willing to recurse during parsing
   121  const maxNestLev int = 1e5
   122  
   123  func incNestLev(p *parser) *parser {
   124  	p.nestLev++
   125  	if p.nestLev > maxNestLev {
   126  		p.error(p.pos, "exceeded max nesting depth")
   127  		panic(bailout{})
   128  	}
   129  	return p
   130  }
   131  
   132  // decNestLev is used to track nesting depth during parsing to prevent stack exhaustion.
   133  // It is used along with incNestLev in a similar fashion to how un and trace are used.
   134  func decNestLev(p *parser) {
   135  	p.nestLev--
   136  }
   137  
   138  // Advance to the next token.
   139  func (p *parser) next0() {
   140  	// Because of one-token look-ahead, print the previous token
   141  	// when tracing as it provides a more readable output. The
   142  	// very first token (!p.pos.IsValid()) is not initialized
   143  	// (it is token.ILLEGAL), so don't print it.
   144  	if p.trace && p.pos.IsValid() {
   145  		s := p.tok.String()
   146  		switch {
   147  		case p.tok.IsLiteral():
   148  			p.printTrace(s, p.lit)
   149  		case p.tok.IsOperator(), p.tok.IsKeyword():
   150  			p.printTrace("\"" + s + "\"")
   151  		default:
   152  			p.printTrace(s)
   153  		}
   154  	}
   155  
   156  	for {
   157  		p.pos, p.tok, p.lit = p.scanner.Scan()
   158  		if p.tok == token.COMMENT {
   159  			if p.top && strings.HasPrefix(p.lit, "//go:build") {
   160  				if x, err := constraint.Parse(p.lit); err == nil {
   161  					p.goVersion = constraint.GoVersion(x)
   162  				}
   163  			}
   164  			if p.mode&ParseComments == 0 {
   165  				continue
   166  			}
   167  		} else {
   168  			if p.tok == token.STRING {
   169  				p.stringEnd = scannerhooks.StringEnd(&p.scanner)
   170  			}
   171  
   172  			// Found a non-comment; top of file is over.
   173  			p.top = false
   174  		}
   175  		break
   176  	}
   177  }
   178  
   179  // lineFor returns the line of pos, ignoring line directive adjustments.
   180  func (p *parser) lineFor(pos token.Pos) int {
   181  	return p.file.PositionFor(pos, false).Line
   182  }
   183  
   184  // Consume a comment and return it and the line on which it ends.
   185  func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
   186  	// /*-style comments may end on a different line than where they start.
   187  	// Scan the comment for '\n' chars and adjust endline accordingly.
   188  	endline = p.lineFor(p.pos)
   189  	if p.lit[1] == '*' {
   190  		// don't use range here - no need to decode Unicode code points
   191  		for i := 0; i < len(p.lit); i++ {
   192  			if p.lit[i] == '\n' {
   193  				endline++
   194  			}
   195  		}
   196  	}
   197  
   198  	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
   199  	p.next0()
   200  
   201  	return
   202  }
   203  
   204  // Consume a group of adjacent comments, add it to the parser's
   205  // comments list, and return it together with the line at which
   206  // the last comment in the group ends. A non-comment token or n
   207  // empty lines terminate a comment group.
   208  func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
   209  	var list []*ast.Comment
   210  	endline = p.lineFor(p.pos)
   211  	for p.tok == token.COMMENT && p.lineFor(p.pos) <= endline+n {
   212  		var comment *ast.Comment
   213  		comment, endline = p.consumeComment()
   214  		list = append(list, comment)
   215  	}
   216  
   217  	// add comment group to the comments list
   218  	comments = &ast.CommentGroup{List: list}
   219  	p.comments = append(p.comments, comments)
   220  
   221  	return
   222  }
   223  
   224  // Advance to the next non-comment token. In the process, collect
   225  // any comment groups encountered, and remember the last lead and
   226  // line comments.
   227  //
   228  // A lead comment is a comment group that starts and ends in a
   229  // line without any other tokens and that is followed by a non-comment
   230  // token on the line immediately after the comment group.
   231  //
   232  // A line comment is a comment group that follows a non-comment
   233  // token on the same line, and that has no tokens after it on the line
   234  // where it ends.
   235  //
   236  // Lead and line comments may be considered documentation that is
   237  // stored in the AST.
   238  func (p *parser) next() {
   239  	p.leadComment = nil
   240  	p.lineComment = nil
   241  	prev := p.pos
   242  	p.next0()
   243  
   244  	if p.tok == token.COMMENT {
   245  		var comment *ast.CommentGroup
   246  		var endline int
   247  
   248  		if p.lineFor(p.pos) == p.lineFor(prev) {
   249  			// The comment is on same line as the previous token; it
   250  			// cannot be a lead comment but may be a line comment.
   251  			comment, endline = p.consumeCommentGroup(0)
   252  			if p.lineFor(p.pos) != endline || p.tok == token.SEMICOLON || p.tok == token.EOF {
   253  				// The next token is on a different line, thus
   254  				// the last comment group is a line comment.
   255  				p.lineComment = comment
   256  			}
   257  		}
   258  
   259  		// consume successor comments, if any
   260  		endline = -1
   261  		for p.tok == token.COMMENT {
   262  			comment, endline = p.consumeCommentGroup(1)
   263  		}
   264  
   265  		if endline+1 == p.lineFor(p.pos) {
   266  			// The next token is following on the line immediately after the
   267  			// comment group, thus the last comment group is a lead comment.
   268  			p.leadComment = comment
   269  		}
   270  	}
   271  }
   272  
   273  // A bailout panic is raised to indicate early termination. pos and msg are
   274  // only populated when bailing out of object resolution.
   275  type bailout struct {
   276  	pos token.Pos
   277  	msg string
   278  }
   279  
   280  func (p *parser) error(pos token.Pos, msg string) {
   281  	if p.trace {
   282  		defer un(trace(p, "error: "+msg))
   283  	}
   284  
   285  	epos := p.file.Position(pos)
   286  
   287  	// If AllErrors is not set, discard errors reported on the same line
   288  	// as the last recorded error and stop parsing if there are more than
   289  	// 10 errors.
   290  	if p.mode&AllErrors == 0 {
   291  		n := len(p.errors)
   292  		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
   293  			return // discard - likely a spurious error
   294  		}
   295  		if n > 10 {
   296  			panic(bailout{})
   297  		}
   298  	}
   299  
   300  	p.errors.Add(epos, msg)
   301  }
   302  
   303  func (p *parser) errorExpected(pos token.Pos, msg string) {
   304  	msg = "expected " + msg
   305  	if pos == p.pos {
   306  		// the error happened at the current position;
   307  		// make the error message more specific
   308  		switch {
   309  		case p.tok == token.SEMICOLON && p.lit == "\n":
   310  			msg += ", found newline"
   311  		case p.tok.IsLiteral():
   312  			// print 123 rather than 'INT', etc.
   313  			msg += ", found " + p.lit
   314  		default:
   315  			msg += ", found '" + p.tok.String() + "'"
   316  		}
   317  	}
   318  	p.error(pos, msg)
   319  }
   320  
   321  func (p *parser) expect(tok token.Token) token.Pos {
   322  	pos := p.pos
   323  	if p.tok != tok {
   324  		p.errorExpected(pos, "'"+tok.String()+"'")
   325  	}
   326  	p.next() // make progress
   327  	return pos
   328  }
   329  
   330  // expect2 is like expect, but it returns an invalid position
   331  // if the expected token is not found.
   332  func (p *parser) expect2(tok token.Token) (pos token.Pos) {
   333  	if p.tok == tok {
   334  		pos = p.pos
   335  	} else {
   336  		p.errorExpected(p.pos, "'"+tok.String()+"'")
   337  	}
   338  	p.next() // make progress
   339  	return
   340  }
   341  
   342  // expectClosing is like expect but provides a better error message
   343  // for the common case of a missing comma before a newline.
   344  func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
   345  	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
   346  		p.error(p.pos, "missing ',' before newline in "+context)
   347  		p.next()
   348  	}
   349  	return p.expect(tok)
   350  }
   351  
   352  // expectSemi consumes a semicolon and returns the applicable line comment.
   353  func (p *parser) expectSemi() (comment *ast.CommentGroup) {
   354  	switch p.tok {
   355  	case token.RPAREN, token.RBRACE:
   356  		return nil // semicolon is optional before a closing ')' or '}'
   357  	case token.COMMA:
   358  		// permit a ',' instead of a ';' but complain
   359  		p.errorExpected(p.pos, "';'")
   360  		fallthrough
   361  	case token.SEMICOLON:
   362  		if p.lit == ";" {
   363  			// explicit semicolon
   364  			p.next()
   365  			comment = p.lineComment // use following comments
   366  		} else {
   367  			// artificial semicolon
   368  			comment = p.lineComment // use preceding comments
   369  			p.next()
   370  		}
   371  		return comment
   372  	default:
   373  		p.errorExpected(p.pos, "';'")
   374  		p.advance(stmtStart)
   375  		return nil
   376  	}
   377  }
   378  
   379  func (p *parser) atComma(context string, follow token.Token) bool {
   380  	if p.tok == token.COMMA {
   381  		return true
   382  	}
   383  	if p.tok != follow {
   384  		msg := "missing ','"
   385  		if p.tok == token.SEMICOLON && p.lit == "\n" {
   386  			msg += " before newline"
   387  		}
   388  		p.error(p.pos, msg+" in "+context)
   389  		return true // "insert" comma and continue
   390  	}
   391  	return false
   392  }
   393  
   394  func assert(cond bool, msg string) {
   395  	if !cond {
   396  		panic("go/parser internal error: " + msg)
   397  	}
   398  }
   399  
   400  // advance consumes tokens until the current token p.tok
   401  // is in the 'to' set, or token.EOF. For error recovery.
   402  func (p *parser) advance(to map[token.Token]bool) {
   403  	for ; p.tok != token.EOF; p.next() {
   404  		if to[p.tok] {
   405  			// Return only if parser made some progress since last
   406  			// sync or if it has not reached 10 advance calls without
   407  			// progress. Otherwise consume at least one token to
   408  			// avoid an endless parser loop (it is possible that
   409  			// both parseOperand and parseStmt call advance and
   410  			// correctly do not advance, thus the need for the
   411  			// invocation limit p.syncCnt).
   412  			if p.pos == p.syncPos && p.syncCnt < 10 {
   413  				p.syncCnt++
   414  				return
   415  			}
   416  			if p.pos > p.syncPos {
   417  				p.syncPos = p.pos
   418  				p.syncCnt = 0
   419  				return
   420  			}
   421  			// Reaching here indicates a parser bug, likely an
   422  			// incorrect token list in this function, but it only
   423  			// leads to skipping of possibly correct code if a
   424  			// previous error is present, and thus is preferred
   425  			// over a non-terminating parse.
   426  		}
   427  	}
   428  }
   429  
   430  var stmtStart = map[token.Token]bool{
   431  	token.BREAK:       true,
   432  	token.CONST:       true,
   433  	token.CONTINUE:    true,
   434  	token.DEFER:       true,
   435  	token.FALLTHROUGH: true,
   436  	token.FOR:         true,
   437  	token.GO:          true,
   438  	token.GOTO:        true,
   439  	token.IF:          true,
   440  	token.RETURN:      true,
   441  	token.SELECT:      true,
   442  	token.SWITCH:      true,
   443  	token.TYPE:        true,
   444  	token.VAR:         true,
   445  }
   446  
   447  var declStart = map[token.Token]bool{
   448  	token.IMPORT: true,
   449  	token.CONST:  true,
   450  	token.TYPE:   true,
   451  	token.VAR:    true,
   452  }
   453  
   454  var exprEnd = map[token.Token]bool{
   455  	token.COMMA:     true,
   456  	token.COLON:     true,
   457  	token.SEMICOLON: true,
   458  	token.RPAREN:    true,
   459  	token.RBRACK:    true,
   460  	token.RBRACE:    true,
   461  }
   462  
   463  // ----------------------------------------------------------------------------
   464  // Identifiers
   465  
   466  func (p *parser) parseIdent() *ast.Ident {
   467  	pos := p.pos
   468  	name := "_"
   469  	if p.tok == token.IDENT {
   470  		name = p.lit
   471  		p.next()
   472  	} else {
   473  		p.expect(token.IDENT) // use expect() error handling
   474  	}
   475  	return &ast.Ident{NamePos: pos, Name: name}
   476  }
   477  
   478  func (p *parser) parseIdentList() (list []*ast.Ident) {
   479  	if p.trace {
   480  		defer un(trace(p, "IdentList"))
   481  	}
   482  
   483  	list = append(list, p.parseIdent())
   484  	for p.tok == token.COMMA {
   485  		p.next()
   486  		list = append(list, p.parseIdent())
   487  	}
   488  
   489  	return
   490  }
   491  
   492  // ----------------------------------------------------------------------------
   493  // Common productions
   494  
   495  // If lhs is set, result list elements which are identifiers are not resolved.
   496  func (p *parser) parseExprList() (list []ast.Expr) {
   497  	if p.trace {
   498  		defer un(trace(p, "ExpressionList"))
   499  	}
   500  
   501  	list = append(list, p.parseExpr())
   502  	for p.tok == token.COMMA {
   503  		p.next()
   504  		list = append(list, p.parseExpr())
   505  	}
   506  
   507  	return
   508  }
   509  
   510  func (p *parser) parseList(inRhs bool) []ast.Expr {
   511  	old := p.inRhs
   512  	p.inRhs = inRhs
   513  	list := p.parseExprList()
   514  	p.inRhs = old
   515  	return list
   516  }
   517  
   518  // ----------------------------------------------------------------------------
   519  // Types
   520  
   521  func (p *parser) parseType() ast.Expr {
   522  	if p.trace {
   523  		defer un(trace(p, "Type"))
   524  	}
   525  
   526  	typ := p.tryIdentOrType()
   527  
   528  	if typ == nil {
   529  		pos := p.pos
   530  		p.errorExpected(pos, "type")
   531  		p.advance(exprEnd)
   532  		return &ast.BadExpr{From: pos, To: p.pos}
   533  	}
   534  
   535  	return typ
   536  }
   537  
   538  func (p *parser) parseQualifiedIdent(ident *ast.Ident) ast.Expr {
   539  	if p.trace {
   540  		defer un(trace(p, "QualifiedIdent"))
   541  	}
   542  
   543  	typ := p.parseTypeName(ident)
   544  	if p.tok == token.LBRACK {
   545  		typ = p.parseTypeInstance(typ)
   546  	}
   547  
   548  	return typ
   549  }
   550  
   551  // If the result is an identifier, it is not resolved.
   552  func (p *parser) parseTypeName(ident *ast.Ident) ast.Expr {
   553  	if p.trace {
   554  		defer un(trace(p, "TypeName"))
   555  	}
   556  
   557  	if ident == nil {
   558  		ident = p.parseIdent()
   559  	}
   560  
   561  	if p.tok == token.PERIOD {
   562  		// ident is a package name
   563  		p.next()
   564  		sel := p.parseIdent()
   565  		return &ast.SelectorExpr{X: ident, Sel: sel}
   566  	}
   567  
   568  	return ident
   569  }
   570  
   571  // "[" has already been consumed, and lbrack is its position.
   572  // If len != nil it is the already consumed array length.
   573  func (p *parser) parseArrayType(lbrack token.Pos, len ast.Expr) *ast.ArrayType {
   574  	if p.trace {
   575  		defer un(trace(p, "ArrayType"))
   576  	}
   577  
   578  	if len == nil {
   579  		p.exprLev++
   580  		// always permit ellipsis for more fault-tolerant parsing
   581  		if p.tok == token.ELLIPSIS {
   582  			len = &ast.Ellipsis{Ellipsis: p.pos}
   583  			p.next()
   584  		} else if p.tok != token.RBRACK {
   585  			len = p.parseRhs()
   586  		}
   587  		p.exprLev--
   588  	}
   589  	if p.tok == token.COMMA {
   590  		// Trailing commas are accepted in type parameter
   591  		// lists but not in array type declarations.
   592  		// Accept for better error handling but complain.
   593  		p.error(p.pos, "unexpected comma; expecting ]")
   594  		p.next()
   595  	}
   596  	p.expect(token.RBRACK)
   597  	elt := p.parseType()
   598  	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
   599  }
   600  
   601  func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Expr) {
   602  	if p.trace {
   603  		defer un(trace(p, "ArrayFieldOrTypeInstance"))
   604  	}
   605  
   606  	lbrack := p.expect(token.LBRACK)
   607  	trailingComma := token.NoPos // if valid, the position of a trailing comma preceding the ']'
   608  	var args []ast.Expr
   609  	if p.tok != token.RBRACK {
   610  		p.exprLev++
   611  		args = append(args, p.parseRhs())
   612  		for p.tok == token.COMMA {
   613  			comma := p.pos
   614  			p.next()
   615  			if p.tok == token.RBRACK {
   616  				trailingComma = comma
   617  				break
   618  			}
   619  			args = append(args, p.parseRhs())
   620  		}
   621  		p.exprLev--
   622  	}
   623  	rbrack := p.expect(token.RBRACK)
   624  
   625  	if len(args) == 0 {
   626  		// x []E
   627  		elt := p.parseType()
   628  		return x, &ast.ArrayType{Lbrack: lbrack, Elt: elt}
   629  	}
   630  
   631  	// x [P]E or x[P]
   632  	if len(args) == 1 {
   633  		elt := p.tryIdentOrType()
   634  		if elt != nil {
   635  			// x [P]E
   636  			if trailingComma.IsValid() {
   637  				// Trailing commas are invalid in array type fields.
   638  				p.error(trailingComma, "unexpected comma; expecting ]")
   639  			}
   640  			return x, &ast.ArrayType{Lbrack: lbrack, Len: args[0], Elt: elt}
   641  		}
   642  	}
   643  
   644  	// x[P], x[P1, P2], ...
   645  	return nil, packIndexExpr(x, lbrack, args, rbrack)
   646  }
   647  
   648  func (p *parser) parseFieldDecl() *ast.Field {
   649  	if p.trace {
   650  		defer un(trace(p, "FieldDecl"))
   651  	}
   652  
   653  	doc := p.leadComment
   654  
   655  	var names []*ast.Ident
   656  	var typ ast.Expr
   657  	switch p.tok {
   658  	case token.IDENT:
   659  		name := p.parseIdent()
   660  		if p.tok == token.PERIOD || p.tok == token.STRING || p.tok == token.SEMICOLON || p.tok == token.RBRACE {
   661  			// embedded type
   662  			typ = name
   663  			if p.tok == token.PERIOD {
   664  				typ = p.parseQualifiedIdent(name)
   665  			}
   666  		} else {
   667  			// name1, name2, ... T
   668  			names = []*ast.Ident{name}
   669  			for p.tok == token.COMMA {
   670  				p.next()
   671  				names = append(names, p.parseIdent())
   672  			}
   673  			// Careful dance: We don't know if we have an embedded instantiated
   674  			// type T[P1, P2, ...] or a field T of array type []E or [P]E.
   675  			if len(names) == 1 && p.tok == token.LBRACK {
   676  				name, typ = p.parseArrayFieldOrTypeInstance(name)
   677  				if name == nil {
   678  					names = nil
   679  				}
   680  			} else {
   681  				// T P
   682  				typ = p.parseType()
   683  			}
   684  		}
   685  	case token.MUL:
   686  		star := p.pos
   687  		p.next()
   688  		if p.tok == token.LPAREN {
   689  			// *(T)
   690  			p.error(p.pos, "cannot parenthesize embedded type")
   691  			p.next()
   692  			typ = p.parseQualifiedIdent(nil)
   693  			// expect closing ')' but no need to complain if missing
   694  			if p.tok == token.RPAREN {
   695  				p.next()
   696  			}
   697  		} else {
   698  			// *T
   699  			typ = p.parseQualifiedIdent(nil)
   700  		}
   701  		typ = &ast.StarExpr{Star: star, X: typ}
   702  
   703  	case token.LPAREN:
   704  		p.error(p.pos, "cannot parenthesize embedded type")
   705  		p.next()
   706  		if p.tok == token.MUL {
   707  			// (*T)
   708  			star := p.pos
   709  			p.next()
   710  			typ = &ast.StarExpr{Star: star, X: p.parseQualifiedIdent(nil)}
   711  		} else {
   712  			// (T)
   713  			typ = p.parseQualifiedIdent(nil)
   714  		}
   715  		// expect closing ')' but no need to complain if missing
   716  		if p.tok == token.RPAREN {
   717  			p.next()
   718  		}
   719  
   720  	default:
   721  		pos := p.pos
   722  		p.errorExpected(pos, "field name or embedded type")
   723  		p.advance(exprEnd)
   724  		typ = &ast.BadExpr{From: pos, To: p.pos}
   725  	}
   726  
   727  	var tag *ast.BasicLit
   728  	if p.tok == token.STRING {
   729  		tag = &ast.BasicLit{ValuePos: p.pos, ValueEnd: p.stringEnd, Kind: p.tok, Value: p.lit}
   730  		p.next()
   731  	}
   732  
   733  	comment := p.expectSemi()
   734  
   735  	field := &ast.Field{Doc: doc, Names: names, Type: typ, Tag: tag, Comment: comment}
   736  	return field
   737  }
   738  
   739  func (p *parser) parseStructType() *ast.StructType {
   740  	if p.trace {
   741  		defer un(trace(p, "StructType"))
   742  	}
   743  
   744  	pos := p.expect(token.STRUCT)
   745  	lbrace := p.expect(token.LBRACE)
   746  	var list []*ast.Field
   747  	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
   748  		// a field declaration cannot start with a '(' but we accept
   749  		// it here for more robust parsing and better error messages
   750  		// (parseFieldDecl will check and complain if necessary)
   751  		list = append(list, p.parseFieldDecl())
   752  	}
   753  	rbrace := p.expect(token.RBRACE)
   754  
   755  	return &ast.StructType{
   756  		Struct: pos,
   757  		Fields: &ast.FieldList{
   758  			Opening: lbrace,
   759  			List:    list,
   760  			Closing: rbrace,
   761  		},
   762  	}
   763  }
   764  
   765  func (p *parser) parsePointerType() *ast.StarExpr {
   766  	if p.trace {
   767  		defer un(trace(p, "PointerType"))
   768  	}
   769  
   770  	star := p.expect(token.MUL)
   771  	base := p.parseType()
   772  
   773  	return &ast.StarExpr{Star: star, X: base}
   774  }
   775  
   776  func (p *parser) parseDotsType() *ast.Ellipsis {
   777  	if p.trace {
   778  		defer un(trace(p, "DotsType"))
   779  	}
   780  
   781  	pos := p.expect(token.ELLIPSIS)
   782  	elt := p.parseType()
   783  
   784  	return &ast.Ellipsis{Ellipsis: pos, Elt: elt}
   785  }
   786  
   787  type field struct {
   788  	name *ast.Ident
   789  	typ  ast.Expr
   790  }
   791  
   792  func (p *parser) parseParamDecl(name *ast.Ident, typeSetsOK bool) (f field) {
   793  	// TODO(rFindley) refactor to be more similar to paramDeclOrNil in the syntax
   794  	// package
   795  	if p.trace {
   796  		defer un(trace(p, "ParamDecl"))
   797  	}
   798  
   799  	ptok := p.tok
   800  	if name != nil {
   801  		p.tok = token.IDENT // force token.IDENT case in switch below
   802  	} else if typeSetsOK && p.tok == token.TILDE {
   803  		// "~" ...
   804  		return field{nil, p.embeddedElem(nil)}
   805  	}
   806  
   807  	switch p.tok {
   808  	case token.IDENT:
   809  		// name
   810  		if name != nil {
   811  			f.name = name
   812  			p.tok = ptok
   813  		} else {
   814  			f.name = p.parseIdent()
   815  		}
   816  		switch p.tok {
   817  		case token.IDENT, token.MUL, token.ARROW, token.FUNC, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
   818  			// name type
   819  			f.typ = p.parseType()
   820  
   821  		case token.LBRACK:
   822  			// name "[" type1, ..., typeN "]" or name "[" n "]" type
   823  			f.name, f.typ = p.parseArrayFieldOrTypeInstance(f.name)
   824  
   825  		case token.ELLIPSIS:
   826  			// name "..." type
   827  			f.typ = p.parseDotsType()
   828  			return // don't allow ...type "|" ...
   829  
   830  		case token.PERIOD:
   831  			// name "." ...
   832  			f.typ = p.parseQualifiedIdent(f.name)
   833  			f.name = nil
   834  
   835  		case token.TILDE:
   836  			if typeSetsOK {
   837  				f.typ = p.embeddedElem(nil)
   838  				return
   839  			}
   840  
   841  		case token.OR:
   842  			if typeSetsOK {
   843  				// name "|" typeset
   844  				f.typ = p.embeddedElem(f.name)
   845  				f.name = nil
   846  				return
   847  			}
   848  		}
   849  
   850  	case token.MUL, token.ARROW, token.FUNC, token.LBRACK, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
   851  		// type
   852  		f.typ = p.parseType()
   853  
   854  	case token.ELLIPSIS:
   855  		// "..." type
   856  		// (always accepted)
   857  		f.typ = p.parseDotsType()
   858  		return // don't allow ...type "|" ...
   859  
   860  	default:
   861  		// TODO(rfindley): this is incorrect in the case of type parameter lists
   862  		//                 (should be "']'" in that case)
   863  		p.errorExpected(p.pos, "')'")
   864  		p.advance(exprEnd)
   865  	}
   866  
   867  	// [name] type "|"
   868  	if typeSetsOK && p.tok == token.OR && f.typ != nil {
   869  		f.typ = p.embeddedElem(f.typ)
   870  	}
   871  
   872  	return
   873  }
   874  
   875  func (p *parser) parseParameterList(name0 *ast.Ident, typ0 ast.Expr, closing token.Token, dddok bool) (params []*ast.Field) {
   876  	if p.trace {
   877  		defer un(trace(p, "ParameterList"))
   878  	}
   879  
   880  	// Type parameters are the only parameter list closed by ']'.
   881  	tparams := closing == token.RBRACK
   882  
   883  	pos0 := p.pos
   884  	if name0 != nil {
   885  		pos0 = name0.Pos()
   886  	} else if typ0 != nil {
   887  		pos0 = typ0.Pos()
   888  	}
   889  
   890  	// Note: The code below matches the corresponding code in the syntax
   891  	//       parser closely. Changes must be reflected in either parser.
   892  	//       For the code to match, we use the local []field list that
   893  	//       corresponds to []syntax.Field. At the end, the list must be
   894  	//       converted into an []*ast.Field.
   895  
   896  	var list []field
   897  	var named int // number of parameters that have an explicit name and type
   898  	var typed int // number of parameters that have an explicit type
   899  
   900  	for name0 != nil || p.tok != closing && p.tok != token.EOF {
   901  		var par field
   902  		if typ0 != nil {
   903  			if tparams {
   904  				typ0 = p.embeddedElem(typ0)
   905  			}
   906  			par = field{name0, typ0}
   907  		} else {
   908  			par = p.parseParamDecl(name0, tparams)
   909  		}
   910  		name0 = nil // 1st name was consumed if present
   911  		typ0 = nil  // 1st typ was consumed if present
   912  		if par.name != nil || par.typ != nil {
   913  			list = append(list, par)
   914  			if par.name != nil && par.typ != nil {
   915  				named++
   916  			}
   917  			if par.typ != nil {
   918  				typed++
   919  			}
   920  		}
   921  		if !p.atComma("parameter list", closing) {
   922  			break
   923  		}
   924  		p.next()
   925  	}
   926  
   927  	if len(list) == 0 {
   928  		return // not uncommon
   929  	}
   930  
   931  	// distribute parameter types (len(list) > 0)
   932  	if named == 0 {
   933  		// all unnamed => found names are type names
   934  		for i := range list {
   935  			par := &list[i]
   936  			if typ := par.name; typ != nil {
   937  				par.typ = typ
   938  				par.name = nil
   939  			}
   940  		}
   941  		if tparams {
   942  			// This is the same error handling as below, adjusted for type parameters only.
   943  			// See comment below for details. (go.dev/issue/64534)
   944  			var errPos token.Pos
   945  			var msg string
   946  			if named == typed /* same as typed == 0 */ {
   947  				errPos = p.pos // position error at closing ]
   948  				msg = "missing type constraint"
   949  			} else {
   950  				errPos = pos0 // position at opening [ or first name
   951  				msg = "missing type parameter name"
   952  				if len(list) == 1 {
   953  					msg += " or invalid array length"
   954  				}
   955  			}
   956  			p.error(errPos, msg)
   957  		}
   958  	} else if named != len(list) {
   959  		// some named or we're in a type parameter list => all must be named
   960  		var errPos token.Pos // left-most error position (or invalid)
   961  		var typ ast.Expr     // current type (from right to left)
   962  		for i := range list {
   963  			if par := &list[len(list)-i-1]; par.typ != nil {
   964  				typ = par.typ
   965  				if par.name == nil {
   966  					errPos = typ.Pos()
   967  					n := ast.NewIdent("_")
   968  					n.NamePos = errPos // correct position
   969  					par.name = n
   970  				}
   971  			} else if typ != nil {
   972  				par.typ = typ
   973  			} else {
   974  				// par.typ == nil && typ == nil => we only have a par.name
   975  				errPos = par.name.Pos()
   976  				par.typ = &ast.BadExpr{From: errPos, To: p.pos}
   977  			}
   978  		}
   979  		if errPos.IsValid() {
   980  			// Not all parameters are named because named != len(list).
   981  			// If named == typed, there must be parameters that have no types.
   982  			// They must be at the end of the parameter list, otherwise types
   983  			// would have been filled in by the right-to-left sweep above and
   984  			// there would be no error.
   985  			// If tparams is set, the parameter list is a type parameter list.
   986  			var msg string
   987  			if named == typed {
   988  				errPos = p.pos // position error at closing token ) or ]
   989  				if tparams {
   990  					msg = "missing type constraint"
   991  				} else {
   992  					msg = "missing parameter type"
   993  				}
   994  			} else {
   995  				if tparams {
   996  					msg = "missing type parameter name"
   997  					// go.dev/issue/60812
   998  					if len(list) == 1 {
   999  						msg += " or invalid array length"
  1000  					}
  1001  				} else {
  1002  					msg = "missing parameter name"
  1003  				}
  1004  			}
  1005  			p.error(errPos, msg)
  1006  		}
  1007  	}
  1008  
  1009  	// check use of ...
  1010  	first := true // only report first occurrence
  1011  	for i, _ := range list {
  1012  		f := &list[i]
  1013  		if t, _ := f.typ.(*ast.Ellipsis); t != nil && (!dddok || i+1 < len(list)) {
  1014  			if first {
  1015  				first = false
  1016  				if dddok {
  1017  					p.error(t.Ellipsis, "can only use ... with final parameter")
  1018  				} else {
  1019  					p.error(t.Ellipsis, "invalid use of ...")
  1020  				}
  1021  			}
  1022  			// use T instead of invalid ...T
  1023  			// TODO(gri) would like to use `f.typ = t.Elt` but that causes problems
  1024  			//           with the resolver in cases of reuse of the same identifier
  1025  			f.typ = &ast.BadExpr{From: t.Pos(), To: t.End()}
  1026  		}
  1027  	}
  1028  
  1029  	// Convert list to []*ast.Field.
  1030  	// If list contains types only, each type gets its own ast.Field.
  1031  	if named == 0 {
  1032  		// parameter list consists of types only
  1033  		for _, par := range list {
  1034  			assert(par.typ != nil, "nil type in unnamed parameter list")
  1035  			params = append(params, &ast.Field{Type: par.typ})
  1036  		}
  1037  		return
  1038  	}
  1039  
  1040  	// If the parameter list consists of named parameters with types,
  1041  	// collect all names with the same types into a single ast.Field.
  1042  	var names []*ast.Ident
  1043  	var typ ast.Expr
  1044  	addParams := func() {
  1045  		assert(typ != nil, "nil type in named parameter list")
  1046  		field := &ast.Field{Names: names, Type: typ}
  1047  		params = append(params, field)
  1048  		names = nil
  1049  	}
  1050  	for _, par := range list {
  1051  		if par.typ != typ {
  1052  			if len(names) > 0 {
  1053  				addParams()
  1054  			}
  1055  			typ = par.typ
  1056  		}
  1057  		names = append(names, par.name)
  1058  	}
  1059  	if len(names) > 0 {
  1060  		addParams()
  1061  	}
  1062  	return
  1063  }
  1064  
  1065  func (p *parser) parseTypeParameters() *ast.FieldList {
  1066  	if p.trace {
  1067  		defer un(trace(p, "TypeParameters"))
  1068  	}
  1069  
  1070  	lbrack := p.expect(token.LBRACK)
  1071  	var list []*ast.Field
  1072  	if p.tok != token.RBRACK {
  1073  		list = p.parseParameterList(nil, nil, token.RBRACK, false)
  1074  	}
  1075  	rbrack := p.expect(token.RBRACK)
  1076  
  1077  	if len(list) == 0 {
  1078  		p.error(rbrack, "empty type parameter list")
  1079  		return nil // avoid follow-on errors
  1080  	}
  1081  
  1082  	return &ast.FieldList{Opening: lbrack, List: list, Closing: rbrack}
  1083  }
  1084  
  1085  func (p *parser) parseParameters(result bool) *ast.FieldList {
  1086  	if p.trace {
  1087  		defer un(trace(p, "Parameters"))
  1088  	}
  1089  
  1090  	if !result || p.tok == token.LPAREN {
  1091  		lparen := p.expect(token.LPAREN)
  1092  		var list []*ast.Field
  1093  		if p.tok != token.RPAREN {
  1094  			list = p.parseParameterList(nil, nil, token.RPAREN, !result)
  1095  		}
  1096  		rparen := p.expect(token.RPAREN)
  1097  		return &ast.FieldList{Opening: lparen, List: list, Closing: rparen}
  1098  	}
  1099  
  1100  	if typ := p.tryIdentOrType(); typ != nil {
  1101  		list := make([]*ast.Field, 1)
  1102  		list[0] = &ast.Field{Type: typ}
  1103  		return &ast.FieldList{List: list}
  1104  	}
  1105  
  1106  	return nil
  1107  }
  1108  
  1109  func (p *parser) parseFuncType() *ast.FuncType {
  1110  	if p.trace {
  1111  		defer un(trace(p, "FuncType"))
  1112  	}
  1113  
  1114  	pos := p.expect(token.FUNC)
  1115  	// accept type parameters for more tolerant parsing but complain
  1116  	if p.tok == token.LBRACK {
  1117  		tparams := p.parseTypeParameters()
  1118  		if tparams != nil {
  1119  			p.error(tparams.Opening, "function type must have no type parameters")
  1120  		}
  1121  	}
  1122  	params := p.parseParameters(false)
  1123  	results := p.parseParameters(true)
  1124  
  1125  	return &ast.FuncType{Func: pos, Params: params, Results: results}
  1126  }
  1127  
  1128  func (p *parser) parseMethodSpec() *ast.Field {
  1129  	if p.trace {
  1130  		defer un(trace(p, "MethodSpec"))
  1131  	}
  1132  
  1133  	doc := p.leadComment
  1134  	var idents []*ast.Ident
  1135  	var typ ast.Expr
  1136  	x := p.parseTypeName(nil)
  1137  	if ident, _ := x.(*ast.Ident); ident != nil {
  1138  		switch {
  1139  		case p.tok == token.LBRACK:
  1140  			// generic method or embedded instantiated type
  1141  			lbrack := p.pos
  1142  			p.next()
  1143  			p.exprLev++
  1144  			x := p.parseExpr()
  1145  			p.exprLev--
  1146  			if name0, _ := x.(*ast.Ident); name0 != nil && p.tok != token.COMMA && p.tok != token.RBRACK {
  1147  				// generic method m[T any]
  1148  				//
  1149  				// Interface methods do not have type parameters. We parse them for a
  1150  				// better error message and improved error recovery.
  1151  				_ = p.parseParameterList(name0, nil, token.RBRACK, false)
  1152  				_ = p.expect(token.RBRACK)
  1153  				p.error(lbrack, "interface method must have no type parameters")
  1154  
  1155  				// TODO(rfindley) refactor to share code with parseFuncType.
  1156  				params := p.parseParameters(false)
  1157  				results := p.parseParameters(true)
  1158  				idents = []*ast.Ident{ident}
  1159  				typ = &ast.FuncType{
  1160  					Func:    token.NoPos,
  1161  					Params:  params,
  1162  					Results: results,
  1163  				}
  1164  			} else {
  1165  				// embedded instantiated type
  1166  				// TODO(rfindley) should resolve all identifiers in x.
  1167  				list := []ast.Expr{x}
  1168  				if p.atComma("type argument list", token.RBRACK) {
  1169  					p.exprLev++
  1170  					p.next()
  1171  					for p.tok != token.RBRACK && p.tok != token.EOF {
  1172  						list = append(list, p.parseType())
  1173  						if !p.atComma("type argument list", token.RBRACK) {
  1174  							break
  1175  						}
  1176  						p.next()
  1177  					}
  1178  					p.exprLev--
  1179  				}
  1180  				rbrack := p.expectClosing(token.RBRACK, "type argument list")
  1181  				typ = packIndexExpr(ident, lbrack, list, rbrack)
  1182  			}
  1183  		case p.tok == token.LPAREN:
  1184  			// ordinary method
  1185  			// TODO(rfindley) refactor to share code with parseFuncType.
  1186  			params := p.parseParameters(false)
  1187  			results := p.parseParameters(true)
  1188  			idents = []*ast.Ident{ident}
  1189  			typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
  1190  		default:
  1191  			// embedded type
  1192  			typ = x
  1193  		}
  1194  	} else {
  1195  		// embedded, possibly instantiated type
  1196  		typ = x
  1197  		if p.tok == token.LBRACK {
  1198  			// embedded instantiated interface
  1199  			typ = p.parseTypeInstance(typ)
  1200  		}
  1201  	}
  1202  
  1203  	// Comment is added at the callsite: the field below may joined with
  1204  	// additional type specs using '|'.
  1205  	// TODO(rfindley) this should be refactored.
  1206  	// TODO(rfindley) add more tests for comment handling.
  1207  	return &ast.Field{Doc: doc, Names: idents, Type: typ}
  1208  }
  1209  
  1210  func (p *parser) embeddedElem(x ast.Expr) ast.Expr {
  1211  	if p.trace {
  1212  		defer un(trace(p, "EmbeddedElem"))
  1213  	}
  1214  	if x == nil {
  1215  		x = p.embeddedTerm()
  1216  	}
  1217  	for p.tok == token.OR {
  1218  		t := new(ast.BinaryExpr)
  1219  		t.OpPos = p.pos
  1220  		t.Op = token.OR
  1221  		p.next()
  1222  		t.X = x
  1223  		t.Y = p.embeddedTerm()
  1224  		x = t
  1225  	}
  1226  	return x
  1227  }
  1228  
  1229  func (p *parser) embeddedTerm() ast.Expr {
  1230  	if p.trace {
  1231  		defer un(trace(p, "EmbeddedTerm"))
  1232  	}
  1233  	if p.tok == token.TILDE {
  1234  		t := new(ast.UnaryExpr)
  1235  		t.OpPos = p.pos
  1236  		t.Op = token.TILDE
  1237  		p.next()
  1238  		t.X = p.parseType()
  1239  		return t
  1240  	}
  1241  
  1242  	t := p.tryIdentOrType()
  1243  	if t == nil {
  1244  		pos := p.pos
  1245  		p.errorExpected(pos, "~ term or type")
  1246  		p.advance(exprEnd)
  1247  		return &ast.BadExpr{From: pos, To: p.pos}
  1248  	}
  1249  
  1250  	return t
  1251  }
  1252  
  1253  func (p *parser) parseInterfaceType() *ast.InterfaceType {
  1254  	if p.trace {
  1255  		defer un(trace(p, "InterfaceType"))
  1256  	}
  1257  
  1258  	pos := p.expect(token.INTERFACE)
  1259  	lbrace := p.expect(token.LBRACE)
  1260  
  1261  	var list []*ast.Field
  1262  
  1263  parseElements:
  1264  	for {
  1265  		switch {
  1266  		case p.tok == token.IDENT:
  1267  			f := p.parseMethodSpec()
  1268  			if f.Names == nil {
  1269  				f.Type = p.embeddedElem(f.Type)
  1270  			}
  1271  			f.Comment = p.expectSemi()
  1272  			list = append(list, f)
  1273  		case p.tok == token.TILDE:
  1274  			typ := p.embeddedElem(nil)
  1275  			comment := p.expectSemi()
  1276  			list = append(list, &ast.Field{Type: typ, Comment: comment})
  1277  		default:
  1278  			if t := p.tryIdentOrType(); t != nil {
  1279  				typ := p.embeddedElem(t)
  1280  				comment := p.expectSemi()
  1281  				list = append(list, &ast.Field{Type: typ, Comment: comment})
  1282  			} else {
  1283  				break parseElements
  1284  			}
  1285  		}
  1286  	}
  1287  
  1288  	// TODO(rfindley): the error produced here could be improved, since we could
  1289  	// accept an identifier, 'type', or a '}' at this point.
  1290  	rbrace := p.expect(token.RBRACE)
  1291  
  1292  	return &ast.InterfaceType{
  1293  		Interface: pos,
  1294  		Methods: &ast.FieldList{
  1295  			Opening: lbrace,
  1296  			List:    list,
  1297  			Closing: rbrace,
  1298  		},
  1299  	}
  1300  }
  1301  
  1302  func (p *parser) parseMapType() *ast.MapType {
  1303  	if p.trace {
  1304  		defer un(trace(p, "MapType"))
  1305  	}
  1306  
  1307  	pos := p.expect(token.MAP)
  1308  	p.expect(token.LBRACK)
  1309  	key := p.parseType()
  1310  	p.expect(token.RBRACK)
  1311  	value := p.parseType()
  1312  
  1313  	return &ast.MapType{Map: pos, Key: key, Value: value}
  1314  }
  1315  
  1316  func (p *parser) parseChanType() *ast.ChanType {
  1317  	if p.trace {
  1318  		defer un(trace(p, "ChanType"))
  1319  	}
  1320  
  1321  	pos := p.pos
  1322  	dir := ast.SEND | ast.RECV
  1323  	var arrow token.Pos
  1324  	if p.tok == token.CHAN {
  1325  		p.next()
  1326  		if p.tok == token.ARROW {
  1327  			arrow = p.pos
  1328  			p.next()
  1329  			dir = ast.SEND
  1330  		}
  1331  	} else {
  1332  		arrow = p.expect(token.ARROW)
  1333  		p.expect(token.CHAN)
  1334  		dir = ast.RECV
  1335  	}
  1336  	value := p.parseType()
  1337  
  1338  	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
  1339  }
  1340  
  1341  func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
  1342  	if p.trace {
  1343  		defer un(trace(p, "TypeInstance"))
  1344  	}
  1345  
  1346  	opening := p.expect(token.LBRACK)
  1347  	p.exprLev++
  1348  	var list []ast.Expr
  1349  	for p.tok != token.RBRACK && p.tok != token.EOF {
  1350  		list = append(list, p.parseType())
  1351  		if !p.atComma("type argument list", token.RBRACK) {
  1352  			break
  1353  		}
  1354  		p.next()
  1355  	}
  1356  	p.exprLev--
  1357  
  1358  	closing := p.expectClosing(token.RBRACK, "type argument list")
  1359  
  1360  	if len(list) == 0 {
  1361  		p.errorExpected(closing, "type argument list")
  1362  		return &ast.IndexExpr{
  1363  			X:      typ,
  1364  			Lbrack: opening,
  1365  			Index:  &ast.BadExpr{From: opening + 1, To: closing},
  1366  			Rbrack: closing,
  1367  		}
  1368  	}
  1369  
  1370  	return packIndexExpr(typ, opening, list, closing)
  1371  }
  1372  
  1373  func (p *parser) tryIdentOrType() ast.Expr {
  1374  	defer decNestLev(incNestLev(p))
  1375  
  1376  	switch p.tok {
  1377  	case token.IDENT:
  1378  		typ := p.parseTypeName(nil)
  1379  		if p.tok == token.LBRACK {
  1380  			typ = p.parseTypeInstance(typ)
  1381  		}
  1382  		return typ
  1383  	case token.LBRACK:
  1384  		lbrack := p.expect(token.LBRACK)
  1385  		return p.parseArrayType(lbrack, nil)
  1386  	case token.STRUCT:
  1387  		return p.parseStructType()
  1388  	case token.MUL:
  1389  		return p.parsePointerType()
  1390  	case token.FUNC:
  1391  		return p.parseFuncType()
  1392  	case token.INTERFACE:
  1393  		return p.parseInterfaceType()
  1394  	case token.MAP:
  1395  		return p.parseMapType()
  1396  	case token.CHAN, token.ARROW:
  1397  		return p.parseChanType()
  1398  	case token.LPAREN:
  1399  		lparen := p.pos
  1400  		p.next()
  1401  		typ := p.parseType()
  1402  		rparen := p.expect(token.RPAREN)
  1403  		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
  1404  	}
  1405  
  1406  	// no type found
  1407  	return nil
  1408  }
  1409  
  1410  // ----------------------------------------------------------------------------
  1411  // Blocks
  1412  
  1413  func (p *parser) parseStmtList() (list []ast.Stmt) {
  1414  	if p.trace {
  1415  		defer un(trace(p, "StatementList"))
  1416  	}
  1417  
  1418  	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
  1419  		list = append(list, p.parseStmt())
  1420  	}
  1421  
  1422  	return
  1423  }
  1424  
  1425  func (p *parser) parseBody() *ast.BlockStmt {
  1426  	if p.trace {
  1427  		defer un(trace(p, "Body"))
  1428  	}
  1429  
  1430  	lbrace := p.expect(token.LBRACE)
  1431  	list := p.parseStmtList()
  1432  	rbrace := p.expect2(token.RBRACE)
  1433  
  1434  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1435  }
  1436  
  1437  func (p *parser) parseBlockStmt() *ast.BlockStmt {
  1438  	if p.trace {
  1439  		defer un(trace(p, "BlockStmt"))
  1440  	}
  1441  
  1442  	lbrace := p.expect(token.LBRACE)
  1443  	list := p.parseStmtList()
  1444  	rbrace := p.expect2(token.RBRACE)
  1445  
  1446  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1447  }
  1448  
  1449  // ----------------------------------------------------------------------------
  1450  // Expressions
  1451  
  1452  func (p *parser) parseFuncTypeOrLit() ast.Expr {
  1453  	if p.trace {
  1454  		defer un(trace(p, "FuncTypeOrLit"))
  1455  	}
  1456  
  1457  	typ := p.parseFuncType()
  1458  	if p.tok != token.LBRACE {
  1459  		// function type only
  1460  		return typ
  1461  	}
  1462  
  1463  	p.exprLev++
  1464  	body := p.parseBody()
  1465  	p.exprLev--
  1466  
  1467  	return &ast.FuncLit{Type: typ, Body: body}
  1468  }
  1469  
  1470  // parseOperand may return an expression or a raw type (incl. array
  1471  // types of the form [...]T). Callers must verify the result.
  1472  func (p *parser) parseOperand() ast.Expr {
  1473  	if p.trace {
  1474  		defer un(trace(p, "Operand"))
  1475  	}
  1476  
  1477  	switch p.tok {
  1478  	case token.IDENT:
  1479  		x := p.parseIdent()
  1480  		return x
  1481  
  1482  	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
  1483  		end := p.pos + token.Pos(len(p.lit))
  1484  		if p.tok == token.STRING {
  1485  			end = p.stringEnd
  1486  		}
  1487  		x := &ast.BasicLit{ValuePos: p.pos, ValueEnd: end, Kind: p.tok, Value: p.lit}
  1488  		p.next()
  1489  		return x
  1490  
  1491  	case token.LPAREN:
  1492  		lparen := p.pos
  1493  		p.next()
  1494  		p.exprLev++
  1495  		x := p.parseRhs() // types may be parenthesized: (some type)
  1496  		p.exprLev--
  1497  		rparen := p.expect(token.RPAREN)
  1498  		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
  1499  
  1500  	case token.FUNC:
  1501  		return p.parseFuncTypeOrLit()
  1502  	}
  1503  
  1504  	if typ := p.tryIdentOrType(); typ != nil { // do not consume trailing type parameters
  1505  		// could be type for composite literal or conversion
  1506  		_, isIdent := typ.(*ast.Ident)
  1507  		assert(!isIdent, "type cannot be identifier")
  1508  		return typ
  1509  	}
  1510  
  1511  	// we have an error
  1512  	pos := p.pos
  1513  	p.errorExpected(pos, "operand")
  1514  	p.advance(stmtStart)
  1515  	return &ast.BadExpr{From: pos, To: p.pos}
  1516  }
  1517  
  1518  func (p *parser) parseSelector(x ast.Expr) ast.Expr {
  1519  	if p.trace {
  1520  		defer un(trace(p, "Selector"))
  1521  	}
  1522  
  1523  	sel := p.parseIdent()
  1524  
  1525  	return &ast.SelectorExpr{X: x, Sel: sel}
  1526  }
  1527  
  1528  func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
  1529  	if p.trace {
  1530  		defer un(trace(p, "TypeAssertion"))
  1531  	}
  1532  
  1533  	lparen := p.expect(token.LPAREN)
  1534  	var typ ast.Expr
  1535  	if p.tok == token.TYPE {
  1536  		// type switch: typ == nil
  1537  		p.next()
  1538  	} else {
  1539  		typ = p.parseType()
  1540  	}
  1541  	rparen := p.expect(token.RPAREN)
  1542  
  1543  	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
  1544  }
  1545  
  1546  func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
  1547  	if p.trace {
  1548  		defer un(trace(p, "parseIndexOrSliceOrInstance"))
  1549  	}
  1550  
  1551  	lbrack := p.expect(token.LBRACK)
  1552  	if p.tok == token.RBRACK {
  1553  		// empty index, slice or index expressions are not permitted;
  1554  		// accept them for parsing tolerance, but complain
  1555  		p.errorExpected(p.pos, "operand")
  1556  		rbrack := p.pos
  1557  		p.next()
  1558  		return &ast.IndexExpr{
  1559  			X:      x,
  1560  			Lbrack: lbrack,
  1561  			Index:  &ast.BadExpr{From: rbrack, To: rbrack},
  1562  			Rbrack: rbrack,
  1563  		}
  1564  	}
  1565  	p.exprLev++
  1566  
  1567  	const N = 3 // change the 3 to 2 to disable 3-index slices
  1568  	var args []ast.Expr
  1569  	var index [N]ast.Expr
  1570  	var colons [N - 1]token.Pos
  1571  	if p.tok != token.COLON {
  1572  		// We can't know if we have an index expression or a type instantiation;
  1573  		// so even if we see a (named) type we are not going to be in type context.
  1574  		index[0] = p.parseRhs()
  1575  	}
  1576  	ncolons := 0
  1577  	switch p.tok {
  1578  	case token.COLON:
  1579  		// slice expression
  1580  		for p.tok == token.COLON && ncolons < len(colons) {
  1581  			colons[ncolons] = p.pos
  1582  			ncolons++
  1583  			p.next()
  1584  			if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
  1585  				index[ncolons] = p.parseRhs()
  1586  			}
  1587  		}
  1588  	case token.COMMA:
  1589  		// instance expression
  1590  		args = append(args, index[0])
  1591  		for p.tok == token.COMMA {
  1592  			p.next()
  1593  			if p.tok != token.RBRACK && p.tok != token.EOF {
  1594  				args = append(args, p.parseType())
  1595  			}
  1596  		}
  1597  	}
  1598  
  1599  	p.exprLev--
  1600  	rbrack := p.expect(token.RBRACK)
  1601  
  1602  	if ncolons > 0 {
  1603  		// slice expression
  1604  		slice3 := false
  1605  		if ncolons == 2 {
  1606  			slice3 = true
  1607  			// Check presence of middle and final index here rather than during type-checking
  1608  			// to prevent erroneous programs from passing through gofmt (was go.dev/issue/7305).
  1609  			if index[1] == nil {
  1610  				p.error(colons[0], "middle index required in 3-index slice")
  1611  				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
  1612  			}
  1613  			if index[2] == nil {
  1614  				p.error(colons[1], "final index required in 3-index slice")
  1615  				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
  1616  			}
  1617  		}
  1618  		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
  1619  	}
  1620  
  1621  	if len(args) == 0 {
  1622  		// index expression
  1623  		return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
  1624  	}
  1625  
  1626  	// instance expression
  1627  	return packIndexExpr(x, lbrack, args, rbrack)
  1628  }
  1629  
  1630  func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
  1631  	if p.trace {
  1632  		defer un(trace(p, "CallOrConversion"))
  1633  	}
  1634  
  1635  	lparen := p.expect(token.LPAREN)
  1636  	p.exprLev++
  1637  	var list []ast.Expr
  1638  	var ellipsis token.Pos
  1639  	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
  1640  		list = append(list, p.parseRhs()) // builtins may expect a type: make(some type, ...)
  1641  		if p.tok == token.ELLIPSIS {
  1642  			ellipsis = p.pos
  1643  			p.next()
  1644  		}
  1645  		if !p.atComma("argument list", token.RPAREN) {
  1646  			break
  1647  		}
  1648  		p.next()
  1649  	}
  1650  	p.exprLev--
  1651  	rparen := p.expectClosing(token.RPAREN, "argument list")
  1652  
  1653  	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
  1654  }
  1655  
  1656  func (p *parser) parseValue() ast.Expr {
  1657  	if p.trace {
  1658  		defer un(trace(p, "Element"))
  1659  	}
  1660  
  1661  	if p.tok == token.LBRACE {
  1662  		return p.parseLiteralValue(nil)
  1663  	}
  1664  
  1665  	x := p.parseExpr()
  1666  
  1667  	return x
  1668  }
  1669  
  1670  func (p *parser) parseElement() ast.Expr {
  1671  	if p.trace {
  1672  		defer un(trace(p, "Element"))
  1673  	}
  1674  
  1675  	x := p.parseValue()
  1676  	if p.tok == token.COLON {
  1677  		colon := p.pos
  1678  		p.next()
  1679  		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue()}
  1680  	}
  1681  
  1682  	return x
  1683  }
  1684  
  1685  func (p *parser) parseElementList() (list []ast.Expr) {
  1686  	if p.trace {
  1687  		defer un(trace(p, "ElementList"))
  1688  	}
  1689  
  1690  	for p.tok != token.RBRACE && p.tok != token.EOF {
  1691  		list = append(list, p.parseElement())
  1692  		if !p.atComma("composite literal", token.RBRACE) {
  1693  			break
  1694  		}
  1695  		p.next()
  1696  	}
  1697  
  1698  	return
  1699  }
  1700  
  1701  func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
  1702  	defer decNestLev(incNestLev(p))
  1703  
  1704  	if p.trace {
  1705  		defer un(trace(p, "LiteralValue"))
  1706  	}
  1707  
  1708  	lbrace := p.expect(token.LBRACE)
  1709  	var elts []ast.Expr
  1710  	p.exprLev++
  1711  	if p.tok != token.RBRACE {
  1712  		elts = p.parseElementList()
  1713  	}
  1714  	p.exprLev--
  1715  	rbrace := p.expectClosing(token.RBRACE, "composite literal")
  1716  	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
  1717  }
  1718  
  1719  func (p *parser) parsePrimaryExpr(x ast.Expr) ast.Expr {
  1720  	if p.trace {
  1721  		defer un(trace(p, "PrimaryExpr"))
  1722  	}
  1723  
  1724  	if x == nil {
  1725  		x = p.parseOperand()
  1726  	}
  1727  	// We track the nesting here rather than at the entry for the function,
  1728  	// since it can iteratively produce a nested output, and we want to
  1729  	// limit how deep a structure we generate.
  1730  	var n int
  1731  	defer func() { p.nestLev -= n }()
  1732  	for n = 1; ; n++ {
  1733  		incNestLev(p)
  1734  		switch p.tok {
  1735  		case token.PERIOD:
  1736  			p.next()
  1737  			switch p.tok {
  1738  			case token.IDENT:
  1739  				x = p.parseSelector(x)
  1740  			case token.LPAREN:
  1741  				x = p.parseTypeAssertion(x)
  1742  			default:
  1743  				pos := p.pos
  1744  				p.errorExpected(pos, "selector or type assertion")
  1745  				// TODO(rFindley) The check for token.RBRACE below is a targeted fix
  1746  				//                to error recovery sufficient to make the x/tools tests to
  1747  				//                pass with the new parsing logic introduced for type
  1748  				//                parameters. Remove this once error recovery has been
  1749  				//                more generally reconsidered.
  1750  				if p.tok != token.RBRACE {
  1751  					p.next() // make progress
  1752  				}
  1753  				sel := &ast.Ident{NamePos: pos, Name: "_"}
  1754  				x = &ast.SelectorExpr{X: x, Sel: sel}
  1755  			}
  1756  		case token.LBRACK:
  1757  			x = p.parseIndexOrSliceOrInstance(x)
  1758  		case token.LPAREN:
  1759  			x = p.parseCallOrConversion(x)
  1760  		case token.LBRACE:
  1761  			// operand may have returned a parenthesized complit
  1762  			// type; accept it but complain if we have a complit
  1763  			t := ast.Unparen(x)
  1764  			// determine if '{' belongs to a composite literal or a block statement
  1765  			switch t.(type) {
  1766  			case *ast.BadExpr, *ast.Ident, *ast.SelectorExpr:
  1767  				if p.exprLev < 0 {
  1768  					return x
  1769  				}
  1770  				// x is possibly a composite literal type
  1771  			case *ast.IndexExpr, *ast.IndexListExpr:
  1772  				if p.exprLev < 0 {
  1773  					return x
  1774  				}
  1775  				// x is possibly a composite literal type
  1776  			case *ast.ArrayType, *ast.StructType, *ast.MapType:
  1777  				// x is a composite literal type
  1778  			default:
  1779  				return x
  1780  			}
  1781  			if t != x {
  1782  				p.error(t.Pos(), "cannot parenthesize type in composite literal")
  1783  				// already progressed, no need to advance
  1784  			}
  1785  			x = p.parseLiteralValue(x)
  1786  		default:
  1787  			return x
  1788  		}
  1789  	}
  1790  }
  1791  
  1792  func (p *parser) parseUnaryExpr() ast.Expr {
  1793  	defer decNestLev(incNestLev(p))
  1794  
  1795  	if p.trace {
  1796  		defer un(trace(p, "UnaryExpr"))
  1797  	}
  1798  
  1799  	switch p.tok {
  1800  	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND, token.TILDE:
  1801  		pos, op := p.pos, p.tok
  1802  		p.next()
  1803  		x := p.parseUnaryExpr()
  1804  		return &ast.UnaryExpr{OpPos: pos, Op: op, X: x}
  1805  
  1806  	case token.ARROW:
  1807  		// channel type or receive expression
  1808  		arrow := p.pos
  1809  		p.next()
  1810  
  1811  		// If the next token is token.CHAN we still don't know if it
  1812  		// is a channel type or a receive operation - we only know
  1813  		// once we have found the end of the unary expression. There
  1814  		// are two cases:
  1815  		//
  1816  		//   <- type  => (<-type) must be channel type
  1817  		//   <- expr  => <-(expr) is a receive from an expression
  1818  		//
  1819  		// In the first case, the arrow must be re-associated with
  1820  		// the channel type parsed already:
  1821  		//
  1822  		//   <- (chan type)    =>  (<-chan type)
  1823  		//   <- (chan<- type)  =>  (<-chan (<-type))
  1824  
  1825  		x := p.parseUnaryExpr()
  1826  
  1827  		// determine which case we have
  1828  		if typ, ok := x.(*ast.ChanType); ok {
  1829  			// (<-type)
  1830  
  1831  			// re-associate position info and <-
  1832  			dir := ast.SEND
  1833  			for ok && dir == ast.SEND {
  1834  				if typ.Dir == ast.RECV {
  1835  					// error: (<-type) is (<-(<-chan T))
  1836  					p.errorExpected(typ.Arrow, "'chan'")
  1837  				}
  1838  				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
  1839  				dir, typ.Dir = typ.Dir, ast.RECV
  1840  				typ, ok = typ.Value.(*ast.ChanType)
  1841  			}
  1842  			if dir == ast.SEND {
  1843  				p.errorExpected(arrow, "channel type")
  1844  			}
  1845  
  1846  			return x
  1847  		}
  1848  
  1849  		// <-(expr)
  1850  		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: x}
  1851  
  1852  	case token.MUL:
  1853  		// pointer type or unary "*" expression
  1854  		pos := p.pos
  1855  		p.next()
  1856  		x := p.parseUnaryExpr()
  1857  		return &ast.StarExpr{Star: pos, X: x}
  1858  	}
  1859  
  1860  	return p.parsePrimaryExpr(nil)
  1861  }
  1862  
  1863  func (p *parser) tokPrec() (token.Token, int) {
  1864  	tok := p.tok
  1865  	if p.inRhs && tok == token.ASSIGN {
  1866  		tok = token.EQL
  1867  	}
  1868  	return tok, tok.Precedence()
  1869  }
  1870  
  1871  // parseBinaryExpr parses a (possibly) binary expression.
  1872  // If x is non-nil, it is used as the left operand.
  1873  //
  1874  // TODO(rfindley): parseBinaryExpr has become overloaded. Consider refactoring.
  1875  func (p *parser) parseBinaryExpr(x ast.Expr, prec1 int) ast.Expr {
  1876  	if p.trace {
  1877  		defer un(trace(p, "BinaryExpr"))
  1878  	}
  1879  
  1880  	if x == nil {
  1881  		x = p.parseUnaryExpr()
  1882  	}
  1883  	// We track the nesting here rather than at the entry for the function,
  1884  	// since it can iteratively produce a nested output, and we want to
  1885  	// limit how deep a structure we generate.
  1886  	var n int
  1887  	defer func() { p.nestLev -= n }()
  1888  	for n = 1; ; n++ {
  1889  		incNestLev(p)
  1890  		op, oprec := p.tokPrec()
  1891  		if oprec < prec1 {
  1892  			return x
  1893  		}
  1894  		pos := p.expect(op)
  1895  		y := p.parseBinaryExpr(nil, oprec+1)
  1896  		x = &ast.BinaryExpr{X: x, OpPos: pos, Op: op, Y: y}
  1897  	}
  1898  }
  1899  
  1900  // The result may be a type or even a raw type ([...]int).
  1901  func (p *parser) parseExpr() ast.Expr {
  1902  	if p.trace {
  1903  		defer un(trace(p, "Expression"))
  1904  	}
  1905  
  1906  	return p.parseBinaryExpr(nil, token.LowestPrec+1)
  1907  }
  1908  
  1909  func (p *parser) parseRhs() ast.Expr {
  1910  	old := p.inRhs
  1911  	p.inRhs = true
  1912  	x := p.parseExpr()
  1913  	p.inRhs = old
  1914  	return x
  1915  }
  1916  
  1917  // ----------------------------------------------------------------------------
  1918  // Statements
  1919  
  1920  // Parsing modes for parseSimpleStmt.
  1921  const (
  1922  	basic = iota
  1923  	labelOk
  1924  	rangeOk
  1925  )
  1926  
  1927  // parseSimpleStmt returns true as 2nd result if it parsed the assignment
  1928  // of a range clause (with mode == rangeOk). The returned statement is an
  1929  // assignment with a right-hand side that is a single unary expression of
  1930  // the form "range x". No guarantees are given for the left-hand side.
  1931  func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
  1932  	if p.trace {
  1933  		defer un(trace(p, "SimpleStmt"))
  1934  	}
  1935  
  1936  	x := p.parseList(false)
  1937  
  1938  	switch p.tok {
  1939  	case
  1940  		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
  1941  		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
  1942  		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
  1943  		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
  1944  		// assignment statement, possibly part of a range clause
  1945  		pos, tok := p.pos, p.tok
  1946  		p.next()
  1947  		var y []ast.Expr
  1948  		isRange := false
  1949  		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
  1950  			pos := p.pos
  1951  			p.next()
  1952  			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  1953  			isRange = true
  1954  		} else {
  1955  			y = p.parseList(true)
  1956  		}
  1957  		return &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}, isRange
  1958  	}
  1959  
  1960  	if len(x) > 1 {
  1961  		p.errorExpected(x[0].Pos(), "1 expression")
  1962  		// continue with first expression
  1963  	}
  1964  
  1965  	switch p.tok {
  1966  	case token.COLON:
  1967  		// labeled statement
  1968  		colon := p.pos
  1969  		p.next()
  1970  		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
  1971  			// Go spec: The scope of a label is the body of the function
  1972  			// in which it is declared and excludes the body of any nested
  1973  			// function.
  1974  			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
  1975  			return stmt, false
  1976  		}
  1977  		// The label declaration typically starts at x[0].Pos(), but the label
  1978  		// declaration may be erroneous due to a token after that position (and
  1979  		// before the ':'). If SpuriousErrors is not set, the (only) error
  1980  		// reported for the line is the illegal label error instead of the token
  1981  		// before the ':' that caused the problem. Thus, use the (latest) colon
  1982  		// position for error reporting.
  1983  		p.error(colon, "illegal label declaration")
  1984  		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
  1985  
  1986  	case token.ARROW:
  1987  		// send statement
  1988  		arrow := p.pos
  1989  		p.next()
  1990  		y := p.parseRhs()
  1991  		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
  1992  
  1993  	case token.INC, token.DEC:
  1994  		// increment or decrement
  1995  		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
  1996  		p.next()
  1997  		return s, false
  1998  	}
  1999  
  2000  	// expression
  2001  	return &ast.ExprStmt{X: x[0]}, false
  2002  }
  2003  
  2004  func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
  2005  	x := p.parseRhs() // could be a conversion: (some type)(x)
  2006  	if t := ast.Unparen(x); t != x {
  2007  		p.error(x.Pos(), fmt.Sprintf("expression in %s must not be parenthesized", callType))
  2008  		x = t
  2009  	}
  2010  	if call, isCall := x.(*ast.CallExpr); isCall {
  2011  		return call
  2012  	}
  2013  	if _, isBad := x.(*ast.BadExpr); !isBad {
  2014  		// only report error if it's a new one
  2015  		p.error(x.End(), fmt.Sprintf("expression in %s must be function call", callType))
  2016  	}
  2017  	return nil
  2018  }
  2019  
  2020  func (p *parser) parseGoStmt() ast.Stmt {
  2021  	if p.trace {
  2022  		defer un(trace(p, "GoStmt"))
  2023  	}
  2024  
  2025  	pos := p.expect(token.GO)
  2026  	call := p.parseCallExpr("go")
  2027  	p.expectSemi()
  2028  	if call == nil {
  2029  		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
  2030  	}
  2031  
  2032  	return &ast.GoStmt{Go: pos, Call: call}
  2033  }
  2034  
  2035  func (p *parser) parseDeferStmt() ast.Stmt {
  2036  	if p.trace {
  2037  		defer un(trace(p, "DeferStmt"))
  2038  	}
  2039  
  2040  	pos := p.expect(token.DEFER)
  2041  	call := p.parseCallExpr("defer")
  2042  	p.expectSemi()
  2043  	if call == nil {
  2044  		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
  2045  	}
  2046  
  2047  	return &ast.DeferStmt{Defer: pos, Call: call}
  2048  }
  2049  
  2050  func (p *parser) parseReturnStmt() *ast.ReturnStmt {
  2051  	if p.trace {
  2052  		defer un(trace(p, "ReturnStmt"))
  2053  	}
  2054  
  2055  	pos := p.pos
  2056  	p.expect(token.RETURN)
  2057  	var x []ast.Expr
  2058  	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
  2059  		x = p.parseList(true)
  2060  	}
  2061  	p.expectSemi()
  2062  
  2063  	return &ast.ReturnStmt{Return: pos, Results: x}
  2064  }
  2065  
  2066  func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
  2067  	if p.trace {
  2068  		defer un(trace(p, "BranchStmt"))
  2069  	}
  2070  
  2071  	pos := p.expect(tok)
  2072  	var label *ast.Ident
  2073  	if tok == token.GOTO || ((tok == token.CONTINUE || tok == token.BREAK) && p.tok == token.IDENT) {
  2074  		label = p.parseIdent()
  2075  	}
  2076  	p.expectSemi()
  2077  
  2078  	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
  2079  }
  2080  
  2081  func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
  2082  	if s == nil {
  2083  		return nil
  2084  	}
  2085  	if es, isExpr := s.(*ast.ExprStmt); isExpr {
  2086  		return es.X
  2087  	}
  2088  	found := "simple statement"
  2089  	if _, isAss := s.(*ast.AssignStmt); isAss {
  2090  		found = "assignment"
  2091  	}
  2092  	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
  2093  	return &ast.BadExpr{From: s.Pos(), To: s.End()}
  2094  }
  2095  
  2096  // parseIfHeader is an adjusted version of parser.header
  2097  // in cmd/compile/internal/syntax/parser.go, which has
  2098  // been tuned for better error handling.
  2099  func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
  2100  	if p.tok == token.LBRACE {
  2101  		p.error(p.pos, "missing condition in if statement")
  2102  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  2103  		return
  2104  	}
  2105  	// p.tok != token.LBRACE
  2106  
  2107  	prevLev := p.exprLev
  2108  	p.exprLev = -1
  2109  
  2110  	if p.tok != token.SEMICOLON {
  2111  		// accept potential variable declaration but complain
  2112  		if p.tok == token.VAR {
  2113  			p.next()
  2114  			p.error(p.pos, "var declaration not allowed in if initializer")
  2115  		}
  2116  		init, _ = p.parseSimpleStmt(basic)
  2117  	}
  2118  
  2119  	var condStmt ast.Stmt
  2120  	var semi struct {
  2121  		pos token.Pos
  2122  		lit string // ";" or "\n"; valid if pos.IsValid()
  2123  	}
  2124  	if p.tok != token.LBRACE {
  2125  		if p.tok == token.SEMICOLON {
  2126  			semi.pos = p.pos
  2127  			semi.lit = p.lit
  2128  			p.next()
  2129  		} else {
  2130  			p.expect(token.SEMICOLON)
  2131  		}
  2132  		if p.tok != token.LBRACE {
  2133  			condStmt, _ = p.parseSimpleStmt(basic)
  2134  		}
  2135  	} else {
  2136  		condStmt = init
  2137  		init = nil
  2138  	}
  2139  
  2140  	if condStmt != nil {
  2141  		cond = p.makeExpr(condStmt, "boolean expression")
  2142  	} else if semi.pos.IsValid() {
  2143  		if semi.lit == "\n" {
  2144  			p.error(semi.pos, "unexpected newline, expecting { after if clause")
  2145  		} else {
  2146  			p.error(semi.pos, "missing condition in if statement")
  2147  		}
  2148  	}
  2149  
  2150  	// make sure we have a valid AST
  2151  	if cond == nil {
  2152  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  2153  	}
  2154  
  2155  	p.exprLev = prevLev
  2156  	return
  2157  }
  2158  
  2159  func (p *parser) parseIfStmt() *ast.IfStmt {
  2160  	defer decNestLev(incNestLev(p))
  2161  
  2162  	if p.trace {
  2163  		defer un(trace(p, "IfStmt"))
  2164  	}
  2165  
  2166  	pos := p.expect(token.IF)
  2167  
  2168  	init, cond := p.parseIfHeader()
  2169  	body := p.parseBlockStmt()
  2170  
  2171  	var else_ ast.Stmt
  2172  	if p.tok == token.ELSE {
  2173  		p.next()
  2174  		switch p.tok {
  2175  		case token.IF:
  2176  			else_ = p.parseIfStmt()
  2177  		case token.LBRACE:
  2178  			else_ = p.parseBlockStmt()
  2179  			p.expectSemi()
  2180  		default:
  2181  			p.errorExpected(p.pos, "if statement or block")
  2182  			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
  2183  		}
  2184  	} else {
  2185  		p.expectSemi()
  2186  	}
  2187  
  2188  	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
  2189  }
  2190  
  2191  func (p *parser) parseCaseClause() *ast.CaseClause {
  2192  	if p.trace {
  2193  		defer un(trace(p, "CaseClause"))
  2194  	}
  2195  
  2196  	pos := p.pos
  2197  	var list []ast.Expr
  2198  	if p.tok == token.CASE {
  2199  		p.next()
  2200  		list = p.parseList(true)
  2201  	} else {
  2202  		p.expect(token.DEFAULT)
  2203  	}
  2204  
  2205  	colon := p.expect(token.COLON)
  2206  	body := p.parseStmtList()
  2207  
  2208  	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
  2209  }
  2210  
  2211  func isTypeSwitchAssert(x ast.Expr) bool {
  2212  	a, ok := x.(*ast.TypeAssertExpr)
  2213  	return ok && a.Type == nil
  2214  }
  2215  
  2216  func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
  2217  	switch t := s.(type) {
  2218  	case *ast.ExprStmt:
  2219  		// x.(type)
  2220  		return isTypeSwitchAssert(t.X)
  2221  	case *ast.AssignStmt:
  2222  		// v := x.(type)
  2223  		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
  2224  			switch t.Tok {
  2225  			case token.ASSIGN:
  2226  				// permit v = x.(type) but complain
  2227  				p.error(t.TokPos, "expected ':=', found '='")
  2228  				fallthrough
  2229  			case token.DEFINE:
  2230  				return true
  2231  			}
  2232  		}
  2233  	}
  2234  	return false
  2235  }
  2236  
  2237  func (p *parser) parseSwitchStmt() ast.Stmt {
  2238  	if p.trace {
  2239  		defer un(trace(p, "SwitchStmt"))
  2240  	}
  2241  
  2242  	pos := p.expect(token.SWITCH)
  2243  
  2244  	var s1, s2 ast.Stmt
  2245  	if p.tok != token.LBRACE {
  2246  		prevLev := p.exprLev
  2247  		p.exprLev = -1
  2248  		if p.tok != token.SEMICOLON {
  2249  			s2, _ = p.parseSimpleStmt(basic)
  2250  		}
  2251  		if p.tok == token.SEMICOLON {
  2252  			p.next()
  2253  			s1 = s2
  2254  			s2 = nil
  2255  			if p.tok != token.LBRACE {
  2256  				// A TypeSwitchGuard may declare a variable in addition
  2257  				// to the variable declared in the initial SimpleStmt.
  2258  				// Introduce extra scope to avoid redeclaration errors:
  2259  				//
  2260  				//	switch t := 0; t := x.(T) { ... }
  2261  				//
  2262  				// (this code is not valid Go because the first t
  2263  				// cannot be accessed and thus is never used, the extra
  2264  				// scope is needed for the correct error message).
  2265  				//
  2266  				// If we don't have a type switch, s2 must be an expression.
  2267  				// Having the extra nested but empty scope won't affect it.
  2268  				s2, _ = p.parseSimpleStmt(basic)
  2269  			}
  2270  		}
  2271  		p.exprLev = prevLev
  2272  	}
  2273  
  2274  	typeSwitch := p.isTypeSwitchGuard(s2)
  2275  	lbrace := p.expect(token.LBRACE)
  2276  	var list []ast.Stmt
  2277  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2278  		list = append(list, p.parseCaseClause())
  2279  	}
  2280  	rbrace := p.expect(token.RBRACE)
  2281  	p.expectSemi()
  2282  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2283  
  2284  	if typeSwitch {
  2285  		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
  2286  	}
  2287  
  2288  	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
  2289  }
  2290  
  2291  func (p *parser) parseCommClause() *ast.CommClause {
  2292  	if p.trace {
  2293  		defer un(trace(p, "CommClause"))
  2294  	}
  2295  
  2296  	pos := p.pos
  2297  	var comm ast.Stmt
  2298  	if p.tok == token.CASE {
  2299  		p.next()
  2300  		lhs := p.parseList(false)
  2301  		if p.tok == token.ARROW {
  2302  			// SendStmt
  2303  			if len(lhs) > 1 {
  2304  				p.errorExpected(lhs[0].Pos(), "1 expression")
  2305  				// continue with first expression
  2306  			}
  2307  			arrow := p.pos
  2308  			p.next()
  2309  			rhs := p.parseRhs()
  2310  			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
  2311  		} else {
  2312  			// RecvStmt
  2313  			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
  2314  				// RecvStmt with assignment
  2315  				if len(lhs) > 2 {
  2316  					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
  2317  					// continue with first two expressions
  2318  					lhs = lhs[0:2]
  2319  				}
  2320  				pos := p.pos
  2321  				p.next()
  2322  				rhs := p.parseRhs()
  2323  				comm = &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
  2324  			} else {
  2325  				// lhs must be single receive operation
  2326  				if len(lhs) > 1 {
  2327  					p.errorExpected(lhs[0].Pos(), "1 expression")
  2328  					// continue with first expression
  2329  				}
  2330  				comm = &ast.ExprStmt{X: lhs[0]}
  2331  			}
  2332  		}
  2333  	} else {
  2334  		p.expect(token.DEFAULT)
  2335  	}
  2336  
  2337  	colon := p.expect(token.COLON)
  2338  	body := p.parseStmtList()
  2339  
  2340  	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
  2341  }
  2342  
  2343  func (p *parser) parseSelectStmt() *ast.SelectStmt {
  2344  	if p.trace {
  2345  		defer un(trace(p, "SelectStmt"))
  2346  	}
  2347  
  2348  	pos := p.expect(token.SELECT)
  2349  	lbrace := p.expect(token.LBRACE)
  2350  	var list []ast.Stmt
  2351  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2352  		list = append(list, p.parseCommClause())
  2353  	}
  2354  	rbrace := p.expect(token.RBRACE)
  2355  	p.expectSemi()
  2356  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2357  
  2358  	return &ast.SelectStmt{Select: pos, Body: body}
  2359  }
  2360  
  2361  func (p *parser) parseForStmt() ast.Stmt {
  2362  	if p.trace {
  2363  		defer un(trace(p, "ForStmt"))
  2364  	}
  2365  
  2366  	pos := p.expect(token.FOR)
  2367  
  2368  	var s1, s2, s3 ast.Stmt
  2369  	var isRange bool
  2370  	if p.tok != token.LBRACE {
  2371  		prevLev := p.exprLev
  2372  		p.exprLev = -1
  2373  		if p.tok != token.SEMICOLON {
  2374  			if p.tok == token.RANGE {
  2375  				// "for range x" (nil lhs in assignment)
  2376  				pos := p.pos
  2377  				p.next()
  2378  				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  2379  				s2 = &ast.AssignStmt{Rhs: y}
  2380  				isRange = true
  2381  			} else {
  2382  				s2, isRange = p.parseSimpleStmt(rangeOk)
  2383  			}
  2384  		}
  2385  		if !isRange && p.tok == token.SEMICOLON {
  2386  			p.next()
  2387  			s1 = s2
  2388  			s2 = nil
  2389  			if p.tok != token.SEMICOLON {
  2390  				s2, _ = p.parseSimpleStmt(basic)
  2391  			}
  2392  			p.expectSemi()
  2393  			if p.tok != token.LBRACE {
  2394  				s3, _ = p.parseSimpleStmt(basic)
  2395  			}
  2396  		}
  2397  		p.exprLev = prevLev
  2398  	}
  2399  
  2400  	body := p.parseBlockStmt()
  2401  	p.expectSemi()
  2402  
  2403  	if isRange {
  2404  		as := s2.(*ast.AssignStmt)
  2405  		// check lhs
  2406  		var key, value ast.Expr
  2407  		switch len(as.Lhs) {
  2408  		case 0:
  2409  			// nothing to do
  2410  		case 1:
  2411  			key = as.Lhs[0]
  2412  		case 2:
  2413  			key, value = as.Lhs[0], as.Lhs[1]
  2414  		default:
  2415  			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
  2416  			return &ast.BadStmt{From: pos, To: body.End()}
  2417  		}
  2418  		// parseSimpleStmt returned a right-hand side that
  2419  		// is a single unary expression of the form "range x"
  2420  		x := as.Rhs[0].(*ast.UnaryExpr).X
  2421  		return &ast.RangeStmt{
  2422  			For:    pos,
  2423  			Key:    key,
  2424  			Value:  value,
  2425  			TokPos: as.TokPos,
  2426  			Tok:    as.Tok,
  2427  			Range:  as.Rhs[0].Pos(),
  2428  			X:      x,
  2429  			Body:   body,
  2430  		}
  2431  	}
  2432  
  2433  	// regular for statement
  2434  	return &ast.ForStmt{
  2435  		For:  pos,
  2436  		Init: s1,
  2437  		Cond: p.makeExpr(s2, "boolean or range expression"),
  2438  		Post: s3,
  2439  		Body: body,
  2440  	}
  2441  }
  2442  
  2443  func (p *parser) parseStmt() (s ast.Stmt) {
  2444  	defer decNestLev(incNestLev(p))
  2445  
  2446  	if p.trace {
  2447  		defer un(trace(p, "Statement"))
  2448  	}
  2449  
  2450  	switch p.tok {
  2451  	case token.CONST, token.TYPE, token.VAR:
  2452  		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
  2453  	case
  2454  		// tokens that may start an expression
  2455  		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
  2456  		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
  2457  		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
  2458  		s, _ = p.parseSimpleStmt(labelOk)
  2459  		// because of the required look-ahead, labeled statements are
  2460  		// parsed by parseSimpleStmt - don't expect a semicolon after
  2461  		// them
  2462  		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
  2463  			p.expectSemi()
  2464  		}
  2465  	case token.GO:
  2466  		s = p.parseGoStmt()
  2467  	case token.DEFER:
  2468  		s = p.parseDeferStmt()
  2469  	case token.RETURN:
  2470  		s = p.parseReturnStmt()
  2471  	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
  2472  		s = p.parseBranchStmt(p.tok)
  2473  	case token.LBRACE:
  2474  		s = p.parseBlockStmt()
  2475  		p.expectSemi()
  2476  	case token.IF:
  2477  		s = p.parseIfStmt()
  2478  	case token.SWITCH:
  2479  		s = p.parseSwitchStmt()
  2480  	case token.SELECT:
  2481  		s = p.parseSelectStmt()
  2482  	case token.FOR:
  2483  		s = p.parseForStmt()
  2484  	case token.SEMICOLON:
  2485  		// Is it ever possible to have an implicit semicolon
  2486  		// producing an empty statement in a valid program?
  2487  		// (handle correctly anyway)
  2488  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
  2489  		p.next()
  2490  	case token.RBRACE:
  2491  		// a semicolon may be omitted before a closing "}"
  2492  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
  2493  	default:
  2494  		// no statement found
  2495  		pos := p.pos
  2496  		p.errorExpected(pos, "statement")
  2497  		p.advance(stmtStart)
  2498  		s = &ast.BadStmt{From: pos, To: p.pos}
  2499  	}
  2500  
  2501  	return
  2502  }
  2503  
  2504  // ----------------------------------------------------------------------------
  2505  // Declarations
  2506  
  2507  type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec
  2508  
  2509  func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2510  	if p.trace {
  2511  		defer un(trace(p, "ImportSpec"))
  2512  	}
  2513  
  2514  	var ident *ast.Ident
  2515  	switch p.tok {
  2516  	case token.IDENT:
  2517  		ident = p.parseIdent()
  2518  	case token.PERIOD:
  2519  		ident = &ast.Ident{NamePos: p.pos, Name: "."}
  2520  		p.next()
  2521  	}
  2522  
  2523  	pos := p.pos
  2524  	end := p.pos
  2525  	var path string
  2526  	if p.tok == token.STRING {
  2527  		path = p.lit
  2528  		end = p.stringEnd
  2529  		p.next()
  2530  	} else if p.tok.IsLiteral() {
  2531  		p.error(pos, "import path must be a string")
  2532  		p.next()
  2533  	} else {
  2534  		p.error(pos, "missing import path")
  2535  		p.advance(exprEnd)
  2536  	}
  2537  	comment := p.expectSemi()
  2538  
  2539  	// collect imports
  2540  	spec := &ast.ImportSpec{
  2541  		Doc:     doc,
  2542  		Name:    ident,
  2543  		Path:    &ast.BasicLit{ValuePos: pos, ValueEnd: end, Kind: token.STRING, Value: path},
  2544  		Comment: comment,
  2545  	}
  2546  	p.imports = append(p.imports, spec)
  2547  
  2548  	return spec
  2549  }
  2550  
  2551  func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
  2552  	if p.trace {
  2553  		defer un(trace(p, keyword.String()+"Spec"))
  2554  	}
  2555  
  2556  	idents := p.parseIdentList()
  2557  	var typ ast.Expr
  2558  	var values []ast.Expr
  2559  	switch keyword {
  2560  	case token.CONST:
  2561  		// always permit optional type and initialization for more tolerant parsing
  2562  		if p.tok != token.EOF && p.tok != token.SEMICOLON && p.tok != token.RPAREN {
  2563  			typ = p.tryIdentOrType()
  2564  			if p.tok == token.ASSIGN {
  2565  				p.next()
  2566  				values = p.parseList(true)
  2567  			}
  2568  		}
  2569  	case token.VAR:
  2570  		if p.tok != token.ASSIGN {
  2571  			typ = p.parseType()
  2572  		}
  2573  		if p.tok == token.ASSIGN {
  2574  			p.next()
  2575  			values = p.parseList(true)
  2576  		}
  2577  	default:
  2578  		panic("unreachable")
  2579  	}
  2580  	comment := p.expectSemi()
  2581  
  2582  	spec := &ast.ValueSpec{
  2583  		Doc:     doc,
  2584  		Names:   idents,
  2585  		Type:    typ,
  2586  		Values:  values,
  2587  		Comment: comment,
  2588  	}
  2589  	return spec
  2590  }
  2591  
  2592  func (p *parser) parseGenericType(spec *ast.TypeSpec, openPos token.Pos, name0 *ast.Ident, typ0 ast.Expr) {
  2593  	if p.trace {
  2594  		defer un(trace(p, "parseGenericType"))
  2595  	}
  2596  
  2597  	list := p.parseParameterList(name0, typ0, token.RBRACK, false)
  2598  	closePos := p.expect(token.RBRACK)
  2599  	spec.TypeParams = &ast.FieldList{Opening: openPos, List: list, Closing: closePos}
  2600  	if p.tok == token.ASSIGN {
  2601  		// type alias
  2602  		spec.Assign = p.pos
  2603  		p.next()
  2604  	}
  2605  	spec.Type = p.parseType()
  2606  }
  2607  
  2608  func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2609  	if p.trace {
  2610  		defer un(trace(p, "TypeSpec"))
  2611  	}
  2612  
  2613  	name := p.parseIdent()
  2614  	spec := &ast.TypeSpec{Doc: doc, Name: name}
  2615  
  2616  	if p.tok == token.LBRACK {
  2617  		// spec.Name "[" ...
  2618  		// array/slice type or type parameter list
  2619  		lbrack := p.pos
  2620  		p.next()
  2621  		if p.tok == token.IDENT {
  2622  			// We may have an array type or a type parameter list.
  2623  			// In either case we expect an expression x (which may
  2624  			// just be a name, or a more complex expression) which
  2625  			// we can analyze further.
  2626  			//
  2627  			// A type parameter list may have a type bound starting
  2628  			// with a "[" as in: P []E. In that case, simply parsing
  2629  			// an expression would lead to an error: P[] is invalid.
  2630  			// But since index or slice expressions are never constant
  2631  			// and thus invalid array length expressions, if the name
  2632  			// is followed by "[" it must be the start of an array or
  2633  			// slice constraint. Only if we don't see a "[" do we
  2634  			// need to parse a full expression. Notably, name <- x
  2635  			// is not a concern because name <- x is a statement and
  2636  			// not an expression.
  2637  			var x ast.Expr = p.parseIdent()
  2638  			if p.tok != token.LBRACK {
  2639  				// To parse the expression starting with name, expand
  2640  				// the call sequence we would get by passing in name
  2641  				// to parser.expr, and pass in name to parsePrimaryExpr.
  2642  				p.exprLev++
  2643  				lhs := p.parsePrimaryExpr(x)
  2644  				x = p.parseBinaryExpr(lhs, token.LowestPrec+1)
  2645  				p.exprLev--
  2646  			}
  2647  			// Analyze expression x. If we can split x into a type parameter
  2648  			// name, possibly followed by a type parameter type, we consider
  2649  			// this the start of a type parameter list, with some caveats:
  2650  			// a single name followed by "]" tilts the decision towards an
  2651  			// array declaration; a type parameter type that could also be
  2652  			// an ordinary expression but which is followed by a comma tilts
  2653  			// the decision towards a type parameter list.
  2654  			if pname, ptype := extractName(x, p.tok == token.COMMA); pname != nil && (ptype != nil || p.tok != token.RBRACK) {
  2655  				// spec.Name "[" pname ...
  2656  				// spec.Name "[" pname ptype ...
  2657  				// spec.Name "[" pname ptype "," ...
  2658  				p.parseGenericType(spec, lbrack, pname, ptype) // ptype may be nil
  2659  			} else {
  2660  				// spec.Name "[" pname "]" ...
  2661  				// spec.Name "[" x ...
  2662  				spec.Type = p.parseArrayType(lbrack, x)
  2663  			}
  2664  		} else {
  2665  			// array type
  2666  			spec.Type = p.parseArrayType(lbrack, nil)
  2667  		}
  2668  	} else {
  2669  		// no type parameters
  2670  		if p.tok == token.ASSIGN {
  2671  			// type alias
  2672  			spec.Assign = p.pos
  2673  			p.next()
  2674  		}
  2675  		spec.Type = p.parseType()
  2676  	}
  2677  
  2678  	spec.Comment = p.expectSemi()
  2679  
  2680  	return spec
  2681  }
  2682  
  2683  // extractName splits the expression x into (name, expr) if syntactically
  2684  // x can be written as name expr. The split only happens if expr is a type
  2685  // element (per the isTypeElem predicate) or if force is set.
  2686  // If x is just a name, the result is (name, nil). If the split succeeds,
  2687  // the result is (name, expr). Otherwise the result is (nil, x).
  2688  // Examples:
  2689  //
  2690  //	x           force    name    expr
  2691  //	------------------------------------
  2692  //	P*[]int     T/F      P       *[]int
  2693  //	P*E         T        P       *E
  2694  //	P*E         F        nil     P*E
  2695  //	P([]int)    T/F      P       ([]int)
  2696  //	P(E)        T        P       (E)
  2697  //	P(E)        F        nil     P(E)
  2698  //	P*E|F|~G    T/F      P       *E|F|~G
  2699  //	P*E|F|G     T        P       *E|F|G
  2700  //	P*E|F|G     F        nil     P*E|F|G
  2701  func extractName(x ast.Expr, force bool) (*ast.Ident, ast.Expr) {
  2702  	switch x := x.(type) {
  2703  	case *ast.Ident:
  2704  		return x, nil
  2705  	case *ast.BinaryExpr:
  2706  		switch x.Op {
  2707  		case token.MUL:
  2708  			if name, _ := x.X.(*ast.Ident); name != nil && (force || isTypeElem(x.Y)) {
  2709  				// x = name *x.Y
  2710  				return name, &ast.StarExpr{Star: x.OpPos, X: x.Y}
  2711  			}
  2712  		case token.OR:
  2713  			if name, lhs := extractName(x.X, force || isTypeElem(x.Y)); name != nil && lhs != nil {
  2714  				// x = name lhs|x.Y
  2715  				op := *x
  2716  				op.X = lhs
  2717  				return name, &op
  2718  			}
  2719  		}
  2720  	case *ast.CallExpr:
  2721  		if name, _ := x.Fun.(*ast.Ident); name != nil {
  2722  			if len(x.Args) == 1 && x.Ellipsis == token.NoPos && (force || isTypeElem(x.Args[0])) {
  2723  				// x = name (x.Args[0])
  2724  				// (Note that the cmd/compile/internal/syntax parser does not care
  2725  				// about syntax tree fidelity and does not preserve parentheses here.)
  2726  				return name, &ast.ParenExpr{
  2727  					Lparen: x.Lparen,
  2728  					X:      x.Args[0],
  2729  					Rparen: x.Rparen,
  2730  				}
  2731  			}
  2732  		}
  2733  	}
  2734  	return nil, x
  2735  }
  2736  
  2737  // isTypeElem reports whether x is a (possibly parenthesized) type element expression.
  2738  // The result is false if x could be a type element OR an ordinary (value) expression.
  2739  func isTypeElem(x ast.Expr) bool {
  2740  	switch x := x.(type) {
  2741  	case *ast.ArrayType, *ast.StructType, *ast.FuncType, *ast.InterfaceType, *ast.MapType, *ast.ChanType:
  2742  		return true
  2743  	case *ast.BinaryExpr:
  2744  		return isTypeElem(x.X) || isTypeElem(x.Y)
  2745  	case *ast.UnaryExpr:
  2746  		return x.Op == token.TILDE
  2747  	case *ast.ParenExpr:
  2748  		return isTypeElem(x.X)
  2749  	}
  2750  	return false
  2751  }
  2752  
  2753  func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
  2754  	if p.trace {
  2755  		defer un(trace(p, "GenDecl("+keyword.String()+")"))
  2756  	}
  2757  
  2758  	doc := p.leadComment
  2759  	pos := p.expect(keyword)
  2760  	var lparen, rparen token.Pos
  2761  	var list []ast.Spec
  2762  	if p.tok == token.LPAREN {
  2763  		lparen = p.pos
  2764  		p.next()
  2765  		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
  2766  			list = append(list, f(p.leadComment, keyword, iota))
  2767  		}
  2768  		rparen = p.expect(token.RPAREN)
  2769  		p.expectSemi()
  2770  	} else {
  2771  		list = append(list, f(nil, keyword, 0))
  2772  	}
  2773  
  2774  	return &ast.GenDecl{
  2775  		Doc:    doc,
  2776  		TokPos: pos,
  2777  		Tok:    keyword,
  2778  		Lparen: lparen,
  2779  		Specs:  list,
  2780  		Rparen: rparen,
  2781  	}
  2782  }
  2783  
  2784  func (p *parser) parseFuncDecl() *ast.FuncDecl {
  2785  	if p.trace {
  2786  		defer un(trace(p, "FunctionDecl"))
  2787  	}
  2788  
  2789  	doc := p.leadComment
  2790  	pos := p.expect(token.FUNC)
  2791  
  2792  	var recv *ast.FieldList
  2793  	if p.tok == token.LPAREN {
  2794  		recv = p.parseParameters(false)
  2795  	}
  2796  
  2797  	ident := p.parseIdent()
  2798  
  2799  	var tparams *ast.FieldList
  2800  	if p.tok == token.LBRACK {
  2801  		tparams = p.parseTypeParameters()
  2802  		if recv != nil && tparams != nil {
  2803  			// Method declarations do not have type parameters. We parse them for a
  2804  			// better error message and improved error recovery.
  2805  			p.error(tparams.Opening, "method must have no type parameters")
  2806  			tparams = nil
  2807  		}
  2808  	}
  2809  	params := p.parseParameters(false)
  2810  	results := p.parseParameters(true)
  2811  
  2812  	var body *ast.BlockStmt
  2813  	switch p.tok {
  2814  	case token.LBRACE:
  2815  		body = p.parseBody()
  2816  		p.expectSemi()
  2817  	case token.SEMICOLON:
  2818  		p.next()
  2819  		if p.tok == token.LBRACE {
  2820  			// opening { of function declaration on next line
  2821  			p.error(p.pos, "unexpected semicolon or newline before {")
  2822  			body = p.parseBody()
  2823  			p.expectSemi()
  2824  		}
  2825  	default:
  2826  		p.expectSemi()
  2827  	}
  2828  
  2829  	decl := &ast.FuncDecl{
  2830  		Doc:  doc,
  2831  		Recv: recv,
  2832  		Name: ident,
  2833  		Type: &ast.FuncType{
  2834  			Func:       pos,
  2835  			TypeParams: tparams,
  2836  			Params:     params,
  2837  			Results:    results,
  2838  		},
  2839  		Body: body,
  2840  	}
  2841  	return decl
  2842  }
  2843  
  2844  func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
  2845  	if p.trace {
  2846  		defer un(trace(p, "Declaration"))
  2847  	}
  2848  
  2849  	var f parseSpecFunction
  2850  	switch p.tok {
  2851  	case token.IMPORT:
  2852  		f = p.parseImportSpec
  2853  
  2854  	case token.CONST, token.VAR:
  2855  		f = p.parseValueSpec
  2856  
  2857  	case token.TYPE:
  2858  		f = p.parseTypeSpec
  2859  
  2860  	case token.FUNC:
  2861  		return p.parseFuncDecl()
  2862  
  2863  	default:
  2864  		pos := p.pos
  2865  		p.errorExpected(pos, "declaration")
  2866  		p.advance(sync)
  2867  		return &ast.BadDecl{From: pos, To: p.pos}
  2868  	}
  2869  
  2870  	return p.parseGenDecl(p.tok, f)
  2871  }
  2872  
  2873  // ----------------------------------------------------------------------------
  2874  // Source files
  2875  
  2876  func (p *parser) parseFile() *ast.File {
  2877  	if p.trace {
  2878  		defer un(trace(p, "File"))
  2879  	}
  2880  
  2881  	// Don't bother parsing the rest if we had errors scanning the first token.
  2882  	// Likely not a Go source file at all.
  2883  	if p.errors.Len() != 0 {
  2884  		return nil
  2885  	}
  2886  
  2887  	// package clause
  2888  	doc := p.leadComment
  2889  	pos := p.expect(token.PACKAGE)
  2890  	// Go spec: The package clause is not a declaration;
  2891  	// the package name does not appear in any scope.
  2892  	ident := p.parseIdent()
  2893  	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
  2894  		p.error(p.pos, "invalid package name _")
  2895  	}
  2896  	p.expectSemi()
  2897  
  2898  	// Don't bother parsing the rest if we had errors parsing the package clause.
  2899  	// Likely not a Go source file at all.
  2900  	if p.errors.Len() != 0 {
  2901  		return nil
  2902  	}
  2903  
  2904  	var decls []ast.Decl
  2905  	if p.mode&PackageClauseOnly == 0 {
  2906  		// import decls
  2907  		for p.tok == token.IMPORT {
  2908  			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
  2909  		}
  2910  
  2911  		if p.mode&ImportsOnly == 0 {
  2912  			// rest of package body
  2913  			prev := token.IMPORT
  2914  			for p.tok != token.EOF {
  2915  				// Continue to accept import declarations for error tolerance, but complain.
  2916  				if p.tok == token.IMPORT && prev != token.IMPORT {
  2917  					p.error(p.pos, "imports must appear before other declarations")
  2918  				}
  2919  				prev = p.tok
  2920  
  2921  				decls = append(decls, p.parseDecl(declStart))
  2922  			}
  2923  		}
  2924  	}
  2925  
  2926  	f := &ast.File{
  2927  		Doc:     doc,
  2928  		Package: pos,
  2929  		Name:    ident,
  2930  		Decls:   decls,
  2931  		// File{Start,End} are set by the defer in the caller.
  2932  		Imports:   p.imports,
  2933  		Comments:  p.comments,
  2934  		GoVersion: p.goVersion,
  2935  	}
  2936  	var declErr func(token.Pos, string)
  2937  	if p.mode&DeclarationErrors != 0 {
  2938  		declErr = p.error
  2939  	}
  2940  	if p.mode&SkipObjectResolution == 0 {
  2941  		resolveFile(f, p.file, declErr)
  2942  	}
  2943  
  2944  	return f
  2945  }
  2946  
  2947  // packIndexExpr returns an IndexExpr x[expr0] or IndexListExpr x[expr0, ...].
  2948  func packIndexExpr(x ast.Expr, lbrack token.Pos, exprs []ast.Expr, rbrack token.Pos) ast.Expr {
  2949  	switch len(exprs) {
  2950  	case 0:
  2951  		panic("internal error: packIndexExpr with empty expr slice")
  2952  	case 1:
  2953  		return &ast.IndexExpr{
  2954  			X:      x,
  2955  			Lbrack: lbrack,
  2956  			Index:  exprs[0],
  2957  			Rbrack: rbrack,
  2958  		}
  2959  	default:
  2960  		return &ast.IndexListExpr{
  2961  			X:       x,
  2962  			Lbrack:  lbrack,
  2963  			Indices: exprs,
  2964  			Rbrack:  rbrack,
  2965  		}
  2966  	}
  2967  }
  2968  

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