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