index.go

     1  // Copyright 2021 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  // This file implements typechecking of index/slice expressions.
     6  
     7  package types
     8  
     9  import (
    10  	"go/ast"
    11  	"go/constant"
    12  	"go/token"
    13  	. "internal/types/errors"
    14  )
    15  
    16  // If e is a valid function instantiation, indexExpr returns true.
    17  // In that case x represents the uninstantiated function value and
    18  // it is the caller's responsibility to instantiate the function.
    19  func (check *Checker) indexExpr(x *operand, e *indexedExpr) (isFuncInst bool) {
    20  	check.exprOrType(x, e.x, true)
    21  	// x may be generic
    22  
    23  	switch x.mode() {
    24  	case invalid:
    25  		check.use(e.indices...)
    26  		return false
    27  
    28  	case typexpr:
    29  		// type instantiation
    30  		x.invalidate()
    31  		// TODO(gri) here we re-evaluate e.X - try to avoid this
    32  		x.typ_ = check.varType(e.orig)
    33  		if isValid(x.typ()) {
    34  			x.mode_ = typexpr
    35  		}
    36  		return false
    37  
    38  	case value:
    39  		if sig, _ := x.typ().Underlying().(*Signature); sig != nil && sig.TypeParams().Len() > 0 {
    40  			// function instantiation
    41  			return true
    42  		}
    43  	}
    44  
    45  	// x should not be generic at this point, but be safe and check
    46  	check.nonGeneric(nil, x)
    47  	if !x.isValid() {
    48  		return false
    49  	}
    50  
    51  	// We cannot index on an incomplete type; make sure it's complete.
    52  	if !check.isComplete(x.typ()) {
    53  		x.invalidate()
    54  		return false
    55  	}
    56  	switch typ := x.typ().Underlying().(type) {
    57  	case *Pointer:
    58  		// Additionally, if x.typ is a pointer to an array type, indexing implicitly dereferences the value, meaning
    59  		// its base type must also be complete.
    60  		if !check.isComplete(typ.base) {
    61  			x.invalidate()
    62  			return false
    63  		}
    64  	case *Map:
    65  		// Lastly, if x.typ is a map type, indexing must produce a value of a complete type, meaning
    66  		// its element type must also be complete.
    67  		if !check.isComplete(typ.elem) {
    68  			x.invalidate()
    69  			return false
    70  		}
    71  	}
    72  
    73  	// ordinary index expression
    74  	valid := false
    75  	length := int64(-1) // valid if >= 0
    76  	switch typ := x.typ().Underlying().(type) {
    77  	case *Basic:
    78  		if isString(typ) {
    79  			valid = true
    80  			if x.mode() == constant_ {
    81  				length = constant.StringLen(x.val)
    82  			}
    83  			// an indexed string always yields a byte value
    84  			// (not a constant) even if the string and the
    85  			// index are constant
    86  			x.mode_ = value
    87  			x.typ_ = universeByte // use 'byte' name
    88  		}
    89  
    90  	case *Array:
    91  		valid = true
    92  		length = typ.len
    93  		if x.mode() != variable {
    94  			x.mode_ = value
    95  		}
    96  		x.typ_ = typ.elem
    97  
    98  	case *Pointer:
    99  		if typ, _ := typ.base.Underlying().(*Array); typ != nil {
   100  			valid = true
   101  			length = typ.len
   102  			x.mode_ = variable
   103  			x.typ_ = typ.elem
   104  		}
   105  
   106  	case *Slice:
   107  		valid = true
   108  		x.mode_ = variable
   109  		x.typ_ = typ.elem
   110  
   111  	case *Map:
   112  		index := check.singleIndex(e)
   113  		if index == nil {
   114  			x.invalidate()
   115  			return false
   116  		}
   117  		var key operand
   118  		check.genericExpr(typ.key, &key, index, nil)
   119  		check.assignment(&key, typ.key, "map index")
   120  		// ok to continue even if indexing failed - map element type is known
   121  		x.mode_ = mapindex
   122  		x.typ_ = typ.elem
   123  		x.expr = e.orig
   124  		return false
   125  
   126  	case *Interface:
   127  		if !isTypeParam(x.typ()) {
   128  			break
   129  		}
   130  		// TODO(gri) report detailed failure cause for better error messages
   131  		var key, elem Type // key != nil: we must have all maps
   132  		mode := variable   // non-maps result mode
   133  		// TODO(gri) factor out closure and use it for non-typeparam cases as well
   134  		if underIs(x.typ(), func(u Type) bool {
   135  			l := int64(-1) // valid if >= 0
   136  			var k, e Type  // k is only set for maps
   137  			switch t := u.(type) {
   138  			case *Basic:
   139  				if isString(t) {
   140  					e = universeByte
   141  					mode = value
   142  				}
   143  			case *Array:
   144  				l = t.len
   145  				e = t.elem
   146  				if x.mode() != variable {
   147  					mode = value
   148  				}
   149  			case *Pointer:
   150  				if t, _ := t.base.Underlying().(*Array); t != nil {
   151  					l = t.len
   152  					e = t.elem
   153  				}
   154  			case *Slice:
   155  				e = t.elem
   156  			case *Map:
   157  				k = t.key
   158  				e = t.elem
   159  			}
   160  			if e == nil {
   161  				return false
   162  			}
   163  			if elem == nil {
   164  				// first type
   165  				length = l
   166  				key, elem = k, e
   167  				return true
   168  			}
   169  			// all map keys must be identical (incl. all nil)
   170  			// (that is, we cannot mix maps with other types)
   171  			if !Identical(key, k) {
   172  				return false
   173  			}
   174  			// all element types must be identical
   175  			if !Identical(elem, e) {
   176  				return false
   177  			}
   178  			// track the minimal length for arrays, if any
   179  			if l >= 0 && l < length {
   180  				length = l
   181  			}
   182  			return true
   183  		}) {
   184  			// For maps, the index expression must be assignable to the map key type.
   185  			if key != nil {
   186  				index := check.singleIndex(e)
   187  				if index == nil {
   188  					x.invalidate()
   189  					return false
   190  				}
   191  				var k operand
   192  				check.genericExpr(key, &k, index, nil)
   193  				check.assignment(&k, key, "map index")
   194  				// ok to continue even if indexing failed - map element type is known
   195  				x.mode_ = mapindex
   196  				x.typ_ = elem
   197  				x.expr = e.orig
   198  				return false
   199  			}
   200  
   201  			// no maps
   202  			valid = true
   203  			x.mode_ = mode
   204  			x.typ_ = elem
   205  		}
   206  	}
   207  
   208  	if !valid {
   209  		// types2 uses the position of '[' for the error
   210  		check.errorf(x, NonIndexableOperand, "cannot index %s", x)
   211  		check.use(e.indices...)
   212  		x.invalidate()
   213  		return false
   214  	}
   215  
   216  	index := check.singleIndex(e)
   217  	if index == nil {
   218  		x.invalidate()
   219  		return false
   220  	}
   221  
   222  	// In pathological (invalid) cases (e.g.: type T1 [][[]T1{}[0][0]]T0)
   223  	// the element type may be accessed before it's set. Make sure we have
   224  	// a valid type.
   225  	if x.typ() == nil {
   226  		x.typ_ = Typ[Invalid]
   227  	}
   228  
   229  	check.index(index, length)
   230  	return false
   231  }
   232  
   233  func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
   234  	check.expr(nil, nil, x, e.X)
   235  	if !x.isValid() {
   236  		check.use(e.Low, e.High, e.Max)
   237  		return
   238  	}
   239  
   240  	// determine common underlying type cu
   241  	var ct, cu Type // type and respective common underlying type
   242  	var hasString bool
   243  	for t, u := range typeset(x.typ()) {
   244  		if u == nil {
   245  			check.errorf(x, NonSliceableOperand, "cannot slice %s: no specific type in %s", x, x.typ())
   246  			cu = nil
   247  			break
   248  		}
   249  
   250  		// Treat strings like byte slices but remember that we saw a string.
   251  		if isString(u) {
   252  			u = NewSlice(universeByte)
   253  			hasString = true
   254  		}
   255  
   256  		// If this is the first type we're seeing, we're done.
   257  		if cu == nil {
   258  			ct, cu = t, u
   259  			continue
   260  		}
   261  
   262  		// Otherwise, the current type must have the same underlying type as all previous types.
   263  		if !Identical(cu, u) {
   264  			check.errorf(x, NonSliceableOperand, "cannot slice %s: %s and %s have different underlying types", x, ct, t)
   265  			cu = nil
   266  			break
   267  		}
   268  	}
   269  	if hasString {
   270  		// If we saw a string, proceed with string type,
   271  		// but don't go from untyped string to string.
   272  		cu = Typ[String]
   273  		if !isTypeParam(x.typ()) {
   274  			cu = x.typ().Underlying() // untyped string remains untyped
   275  		}
   276  	}
   277  
   278  	// Note that we don't permit slice expressions where x is a type expression, so we don't check for that here.
   279  	// However, if x.typ is a pointer to an array type, slicing implicitly dereferences the value, meaning
   280  	// its base type must also be complete.
   281  	if p, ok := x.typ().Underlying().(*Pointer); ok && !check.isComplete(p.base) {
   282  		x.invalidate()
   283  		return
   284  	}
   285  
   286  	valid := false
   287  	length := int64(-1) // valid if >= 0
   288  	switch u := cu.(type) {
   289  	case nil:
   290  		// error reported above
   291  		x.invalidate()
   292  		return
   293  
   294  	case *Basic:
   295  		if isString(u) {
   296  			if e.Slice3 {
   297  				at := e.Max
   298  				if at == nil {
   299  					at = e // e.Index[2] should be present but be careful
   300  				}
   301  				check.error(at, InvalidSliceExpr, invalidOp+"3-index slice of string")
   302  				x.invalidate()
   303  				return
   304  			}
   305  			valid = true
   306  			if x.mode() == constant_ {
   307  				length = constant.StringLen(x.val)
   308  			}
   309  			// spec: "For untyped string operands the result
   310  			// is a non-constant value of type string."
   311  			if isUntyped(x.typ()) {
   312  				x.typ_ = Typ[String]
   313  			}
   314  		}
   315  
   316  	case *Array:
   317  		valid = true
   318  		length = u.len
   319  		if x.mode() != variable {
   320  			check.errorf(x, NonSliceableOperand, "cannot slice unaddressable value %s", x)
   321  			x.invalidate()
   322  			return
   323  		}
   324  		x.typ_ = &Slice{elem: u.elem}
   325  
   326  	case *Pointer:
   327  		if u, _ := u.base.Underlying().(*Array); u != nil {
   328  			valid = true
   329  			length = u.len
   330  			x.typ_ = &Slice{elem: u.elem}
   331  		}
   332  
   333  	case *Slice:
   334  		valid = true
   335  		// x.typ doesn't change
   336  	}
   337  
   338  	if !valid {
   339  		check.errorf(x, NonSliceableOperand, "cannot slice %s", x)
   340  		x.invalidate()
   341  		return
   342  	}
   343  
   344  	x.mode_ = value
   345  
   346  	// spec: "Only the first index may be omitted; it defaults to 0."
   347  	if e.Slice3 && (e.High == nil || e.Max == nil) {
   348  		check.error(inNode(e, e.Rbrack), InvalidSyntaxTree, "2nd and 3rd index required in 3-index slice")
   349  		x.invalidate()
   350  		return
   351  	}
   352  
   353  	// check indices
   354  	var ind [3]int64
   355  	for i, expr := range []ast.Expr{e.Low, e.High, e.Max} {
   356  		x := int64(-1)
   357  		switch {
   358  		case expr != nil:
   359  			// The "capacity" is only known statically for strings, arrays,
   360  			// and pointers to arrays, and it is the same as the length for
   361  			// those types.
   362  			max := int64(-1)
   363  			if length >= 0 {
   364  				max = length + 1
   365  			}
   366  			if _, v := check.index(expr, max); v >= 0 {
   367  				x = v
   368  			}
   369  		case i == 0:
   370  			// default is 0 for the first index
   371  			x = 0
   372  		case length >= 0:
   373  			// default is length (== capacity) otherwise
   374  			x = length
   375  		}
   376  		ind[i] = x
   377  	}
   378  
   379  	// constant indices must be in range
   380  	// (check.index already checks that existing indices >= 0)
   381  L:
   382  	for i, x := range ind[:len(ind)-1] {
   383  		if x > 0 {
   384  			for j, y := range ind[i+1:] {
   385  				if y >= 0 && y < x {
   386  					// The value y corresponds to the expression e.Index[i+1+j].
   387  					// Because y >= 0, it must have been set from the expression
   388  					// when checking indices and thus e.Index[i+1+j] is not nil.
   389  					at := []ast.Expr{e.Low, e.High, e.Max}[i+1+j]
   390  					check.errorf(at, SwappedSliceIndices, "invalid slice indices: %d < %d", y, x)
   391  					break L // only report one error, ok to continue
   392  				}
   393  			}
   394  		}
   395  	}
   396  }
   397  
   398  // singleIndex returns the (single) index from the index expression e.
   399  // If the index is missing, or if there are multiple indices, an error
   400  // is reported and the result is nil.
   401  func (check *Checker) singleIndex(expr *indexedExpr) ast.Expr {
   402  	if len(expr.indices) == 0 {
   403  		check.errorf(expr.orig, InvalidSyntaxTree, "index expression %v with 0 indices", expr)
   404  		return nil
   405  	}
   406  	if len(expr.indices) > 1 {
   407  		// TODO(rFindley) should this get a distinct error code?
   408  		check.error(expr.indices[1], InvalidIndex, invalidOp+"more than one index")
   409  	}
   410  	return expr.indices[0]
   411  }
   412  
   413  // index checks an index expression for validity.
   414  // If max >= 0, it is the upper bound for index.
   415  // If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.
   416  // If the result val >= 0, index is valid and val is its constant int value.
   417  func (check *Checker) index(index ast.Expr, max int64) (typ Type, val int64) {
   418  	typ = Typ[Invalid]
   419  	val = -1
   420  
   421  	var x operand
   422  	check.expr(nil, nil, &x, index)
   423  	if !check.isValidIndex(&x, InvalidIndex, "index", false) {
   424  		return
   425  	}
   426  
   427  	if x.mode() != constant_ {
   428  		return x.typ(), -1
   429  	}
   430  
   431  	if x.val.Kind() == constant.Unknown {
   432  		return
   433  	}
   434  
   435  	v, ok := constant.Int64Val(x.val)
   436  	assert(ok)
   437  	if max >= 0 && v >= max {
   438  		check.errorf(&x, InvalidIndex, invalidArg+"index %s out of bounds [0:%d]", x.val.String(), max)
   439  		return
   440  	}
   441  
   442  	// 0 <= v [ && v < max ]
   443  	return x.typ(), v
   444  }
   445  
   446  func (check *Checker) isValidIndex(x *operand, code Code, what string, allowNegative bool) bool {
   447  	if !x.isValid() {
   448  		return false
   449  	}
   450  
   451  	// spec: "a constant index that is untyped is given type int"
   452  	check.convertUntyped(x, Typ[Int])
   453  	if !x.isValid() {
   454  		return false
   455  	}
   456  
   457  	// spec: "the index x must be of integer type or an untyped constant"
   458  	if !allInteger(x.typ()) {
   459  		check.errorf(x, code, invalidArg+"%s %s must be integer", what, x)
   460  		return false
   461  	}
   462  
   463  	if x.mode() == constant_ {
   464  		// spec: "a constant index must be non-negative ..."
   465  		if !allowNegative && constant.Sign(x.val) < 0 {
   466  			check.errorf(x, code, invalidArg+"%s %s must not be negative", what, x)
   467  			return false
   468  		}
   469  
   470  		// spec: "... and representable by a value of type int"
   471  		if !representableConst(x.val, check, Typ[Int], &x.val) {
   472  			check.errorf(x, code, invalidArg+"%s %s overflows int", what, x)
   473  			return false
   474  		}
   475  	}
   476  
   477  	return true
   478  }
   479  
   480  // indexedExpr wraps an ast.IndexExpr or ast.IndexListExpr.
   481  //
   482  // Orig holds the original ast.Expr from which this indexedExpr was derived.
   483  //
   484  // Note: indexedExpr (intentionally) does not wrap ast.Expr, as that leads to
   485  // accidental misuse such as encountered in golang/go#63933.
   486  //
   487  // TODO(rfindley): remove this helper, in favor of just having a helper
   488  // function that returns indices.
   489  type indexedExpr struct {
   490  	orig    ast.Expr   // the wrapped expr, which may be distinct from the IndexListExpr below.
   491  	x       ast.Expr   // expression
   492  	lbrack  token.Pos  // position of "["
   493  	indices []ast.Expr // index expressions
   494  	rbrack  token.Pos  // position of "]"
   495  }
   496  
   497  func (x *indexedExpr) Pos() token.Pos {
   498  	return x.orig.Pos()
   499  }
   500  
   501  func unpackIndexedExpr(n ast.Node) *indexedExpr {
   502  	switch e := n.(type) {
   503  	case *ast.IndexExpr:
   504  		return &indexedExpr{
   505  			orig:    e,
   506  			x:       e.X,
   507  			lbrack:  e.Lbrack,
   508  			indices: []ast.Expr{e.Index},
   509  			rbrack:  e.Rbrack,
   510  		}
   511  	case *ast.IndexListExpr:
   512  		return &indexedExpr{
   513  			orig:    e,
   514  			x:       e.X,
   515  			lbrack:  e.Lbrack,
   516  			indices: e.Indices,
   517  			rbrack:  e.Rbrack,
   518  		}
   519  	}
   520  	return nil
   521  }
   522
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