src/go/types/index.go - go - Git at Google

 // Copyright 2021 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // This file implements typechecking of index/slice expressions.

 package types

 import (
 	"go/ast"
 	"go/constant"
 	"go/token"
 	. "internal/types/errors"
 )

 // If e is a valid function instantiation, indexExpr returns true.
 // In that case x represents the uninstantiated function value and
 // it is the caller's responsibility to instantiate the function.
 func (check *Checker) indexExpr(x *operand, e *indexedExpr) (isFuncInst bool) {
 	check.exprOrType(x, e.x, true)
 	// x may be generic

 	switch x.mode {
 	case invalid:
 		check.use(e.indices...)
 		return false

 	case typexpr:
 		// type instantiation
 		x.mode = invalid
 		// TODO(gri) here we re-evaluate e.X - try to avoid this
 		x.typ = check.varType(e.orig)
 		if isValid(x.typ) {
 			x.mode = typexpr
 		}
 		return false

 	case value:
 		if sig, _ := under(x.typ).(*Signature); sig != nil && sig.TypeParams().Len() > 0 {
 			// function instantiation
 			return true
 		}
 	}

 	// x should not be generic at this point, but be safe and check
 	check.nonGeneric(nil, x)
 	if x.mode == invalid {
 		return false
 	}

 	// ordinary index expression
 	valid := false
 	length := int64(-1) // valid if >= 0
 	switch typ := under(x.typ).(type) {
 	case *Basic:
 		if isString(typ) {
 			valid = true
 			if x.mode == constant_ {
 				length = int64(len(constant.StringVal(x.val)))
 			}
 			// an indexed string always yields a byte value
 			// (not a constant) even if the string and the
 			// index are constant
 			x.mode = value
 			x.typ = universeByte // use 'byte' name
 		}

 	case *Array:
 		valid = true
 		length = typ.len
 		if x.mode != variable {
 			x.mode = value
 		}
 		x.typ = typ.elem

 	case *Pointer:
 		if typ, _ := under(typ.base).(*Array); typ != nil {
 			valid = true
 			length = typ.len
 			x.mode = variable
 			x.typ = typ.elem
 		}

 	case *Slice:
 		valid = true
 		x.mode = variable
 		x.typ = typ.elem

 	case *Map:
 		index := check.singleIndex(e)
 		if index == nil {
 			x.mode = invalid
 			return false
 		}
 		var key operand
 		check.expr(nil, &key, index)
 		check.assignment(&key, typ.key, "map index")
 		// ok to continue even if indexing failed - map element type is known
 		x.mode = mapindex
 		x.typ = typ.elem
 		x.expr = e.orig
 		return false

 	case *Interface:
 		if !isTypeParam(x.typ) {
 			break
 		}
 		// TODO(gri) report detailed failure cause for better error messages
 		var key, elem Type // key != nil: we must have all maps
 		mode := variable   // non-maps result mode
 		// TODO(gri) factor out closure and use it for non-typeparam cases as well
 		if underIs(x.typ, func(u Type) bool {
 			l := int64(-1) // valid if >= 0
 			var k, e Type  // k is only set for maps
 			switch t := u.(type) {
 			case *Basic:
 				if isString(t) {
 					e = universeByte
 					mode = value
 				}
 			case *Array:
 				l = t.len
 				e = t.elem
 				if x.mode != variable {
 					mode = value
 				}
 			case *Pointer:
 				if t, _ := under(t.base).(*Array); t != nil {
 					l = t.len
 					e = t.elem
 				}
 			case *Slice:
 				e = t.elem
 			case *Map:
 				k = t.key
 				e = t.elem
 			}
 			if e == nil {
 				return false
 			}
 			if elem == nil {
 				// first type
 				length = l
 				key, elem = k, e
 				return true
 			}
 			// all map keys must be identical (incl. all nil)
 			// (that is, we cannot mix maps with other types)
 			if !Identical(key, k) {
 				return false
 			}
 			// all element types must be identical
 			if !Identical(elem, e) {
 				return false
 			}
 			// track the minimal length for arrays, if any
 			if l >= 0 && l < length {
 				length = l
 			}
 			return true
 		}) {
 			// For maps, the index expression must be assignable to the map key type.
 			if key != nil {
 				index := check.singleIndex(e)
 				if index == nil {
 					x.mode = invalid
 					return false
 				}
 				var k operand
 				check.expr(nil, &k, index)
 				check.assignment(&k, key, "map index")
 				// ok to continue even if indexing failed - map element type is known
 				x.mode = mapindex
 				x.typ = elem
 				x.expr = e.orig
 				return false
 			}

 			// no maps
 			valid = true
 			x.mode = mode
 			x.typ = elem
 		}
 	}

 	if !valid {
 		// types2 uses the position of '[' for the error
 		check.errorf(x, NonIndexableOperand, "cannot index %s", x)
 		check.use(e.indices...)
 		x.mode = invalid
 		return false
 	}

 	index := check.singleIndex(e)
 	if index == nil {
 		x.mode = invalid
 		return false
 	}

 	// In pathological (invalid) cases (e.g.: type T1 [][[]T1{}[0][0]]T0)
 	// the element type may be accessed before it's set. Make sure we have
 	// a valid type.
 	if x.typ == nil {
 		x.typ = Typ[Invalid]
 	}

 	check.index(index, length)
 	return false
 }

 func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
 	check.expr(nil, x, e.X)
 	if x.mode == invalid {
 		check.use(e.Low, e.High, e.Max)
 		return
 	}

 	// determine common underlying type cu
 	var ct, cu Type // type and respective common underlying type
 	var hasString bool
 	typeset(x.typ, func(t, u Type) bool {
 		if u == nil {
 			check.errorf(x, NonSliceableOperand, "cannot slice %s: no specific type in %s", x, x.typ)
 			cu = nil
 			return false
 		}

 		// Treat strings like byte slices but remember that we saw a string.
 		if isString(u) {
 			u = NewSlice(universeByte)
 			hasString = true
 		}

 		// If this is the first type we're seeing, we're done.
 		if cu == nil {
 			ct, cu = t, u
 			return true
 		}

 		// Otherwise, the current type must have the same underlying type as all previous types.
 		if !Identical(cu, u) {
 			check.errorf(x, NonSliceableOperand, "cannot slice %s: %s and %s have different underlying types", x, ct, t)
 			cu = nil
 			return false
 		}

 		return true
 	})
 	if hasString {
 		// If we saw a string, proceed with string type,
 		// but don't go from untyped string to string.
 		cu = Typ[String]
 		if !isTypeParam(x.typ) {
 			cu = under(x.typ) // untyped string remains untyped
 		}
 	}

 	valid := false
 	length := int64(-1) // valid if >= 0
 	switch u := cu.(type) {
 	case nil:
 		// error reported above
 		x.mode = invalid
 		return

 	case *Basic:
 		if isString(u) {
 			if e.Slice3 {
 				at := e.Max
 				if at == nil {
 					at = e // e.Index[2] should be present but be careful
 				}
 				check.error(at, InvalidSliceExpr, invalidOp+"3-index slice of string")
 				x.mode = invalid
 				return
 			}
 			valid = true
 			if x.mode == constant_ {
 				length = int64(len(constant.StringVal(x.val)))
 			}
 			// spec: "For untyped string operands the result
 			// is a non-constant value of type string."
 			if isUntyped(x.typ) {
 				x.typ = Typ[String]
 			}
 		}

 	case *Array:
 		valid = true
 		length = u.len
 		if x.mode != variable {
 			check.errorf(x, NonSliceableOperand, "cannot slice unaddressable value %s", x)
 			x.mode = invalid
 			return
 		}
 		x.typ = &Slice{elem: u.elem}

 	case *Pointer:
 		if u, _ := under(u.base).(*Array); u != nil {
 			valid = true
 			length = u.len
 			x.typ = &Slice{elem: u.elem}
 		}

 	case *Slice:
 		valid = true
 		// x.typ doesn't change
 	}

 	if !valid {
 		check.errorf(x, NonSliceableOperand, "cannot slice %s", x)
 		x.mode = invalid
 		return
 	}

 	x.mode = value

 	// spec: "Only the first index may be omitted; it defaults to 0."
 	if e.Slice3 && (e.High == nil || e.Max == nil) {
 		check.error(inNode(e, e.Rbrack), InvalidSyntaxTree, "2nd and 3rd index required in 3-index slice")
 		x.mode = invalid
 		return
 	}

 	// check indices
 	var ind [3]int64
 	for i, expr := range []ast.Expr{e.Low, e.High, e.Max} {
 		x := int64(-1)
 		switch {
 		case expr != nil:
 			// The "capacity" is only known statically for strings, arrays,
 			// and pointers to arrays, and it is the same as the length for
 			// those types.
 			max := int64(-1)
 			if length >= 0 {
 				max = length + 1
 			}
 			if _, v := check.index(expr, max); v >= 0 {
 				x = v
 			}
 		case i == 0:
 			// default is 0 for the first index
 			x = 0
 		case length >= 0:
 			// default is length (== capacity) otherwise
 			x = length
 		}
 		ind[i] = x
 	}

 	// constant indices must be in range
 	// (check.index already checks that existing indices >= 0)
 L:
 	for i, x := range ind[:len(ind)-1] {
 		if x > 0 {
 			for j, y := range ind[i+1:] {
 				if y >= 0 && y < x {
 					// The value y corresponds to the expression e.Index[i+1+j].
 					// Because y >= 0, it must have been set from the expression
 					// when checking indices and thus e.Index[i+1+j] is not nil.
 					at := []ast.Expr{e.Low, e.High, e.Max}[i+1+j]
 					check.errorf(at, SwappedSliceIndices, "invalid slice indices: %d < %d", y, x)
 					break L // only report one error, ok to continue
 				}
 			}
 		}
 	}
 }

 // singleIndex returns the (single) index from the index expression e.
 // If the index is missing, or if there are multiple indices, an error
 // is reported and the result is nil.
 func (check *Checker) singleIndex(expr *indexedExpr) ast.Expr {
 	if len(expr.indices) == 0 {
 		check.errorf(expr.orig, InvalidSyntaxTree, "index expression %v with 0 indices", expr)
 		return nil
 	}
 	if len(expr.indices) > 1 {
 		// TODO(rFindley) should this get a distinct error code?
 		check.error(expr.indices[1], InvalidIndex, invalidOp+"more than one index")
 	}
 	return expr.indices[0]
 }

 // index checks an index expression for validity.
 // If max >= 0, it is the upper bound for index.
 // If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.
 // If the result val >= 0, index is valid and val is its constant int value.
 func (check *Checker) index(index ast.Expr, max int64) (typ Type, val int64) {
 	typ = Typ[Invalid]
 	val = -1

 	var x operand
 	check.expr(nil, &x, index)
 	if !check.isValidIndex(&x, InvalidIndex, "index", false) {
 		return
 	}

 	if x.mode != constant_ {
 		return x.typ, -1
 	}

 	if x.val.Kind() == constant.Unknown {
 		return
 	}

 	v, ok := constant.Int64Val(x.val)
 	assert(ok)
 	if max >= 0 && v >= max {
 		check.errorf(&x, InvalidIndex, invalidArg+"index %s out of bounds [0:%d]", x.val.String(), max)
 		return
 	}

 	// 0 <= v [ && v < max ]
 	return x.typ, v
 }

 func (check *Checker) isValidIndex(x *operand, code Code, what string, allowNegative bool) bool {
 	if x.mode == invalid {
 		return false
 	}

 	// spec: "a constant index that is untyped is given type int"
 	check.convertUntyped(x, Typ[Int])
 	if x.mode == invalid {
 		return false
 	}

 	// spec: "the index x must be of integer type or an untyped constant"
 	if !allInteger(x.typ) {
 		check.errorf(x, code, invalidArg+"%s %s must be integer", what, x)
 		return false
 	}

 	if x.mode == constant_ {
 		// spec: "a constant index must be non-negative ..."
 		if !allowNegative && constant.Sign(x.val) < 0 {
 			check.errorf(x, code, invalidArg+"%s %s must not be negative", what, x)
 			return false
 		}

 		// spec: "... and representable by a value of type int"
 		if !representableConst(x.val, check, Typ[Int], &x.val) {
 			check.errorf(x, code, invalidArg+"%s %s overflows int", what, x)
 			return false
 		}
 	}

 	return true
 }

 // indexedExpr wraps an ast.IndexExpr or ast.IndexListExpr.
 //
 // Orig holds the original ast.Expr from which this indexedExpr was derived.
 //
 // Note: indexedExpr (intentionally) does not wrap ast.Expr, as that leads to
 // accidental misuse such as encountered in golang/go#63933.
 //
 // TODO(rfindley): remove this helper, in favor of just having a helper
 // function that returns indices.
 type indexedExpr struct {
 	orig    ast.Expr   // the wrapped expr, which may be distinct from the IndexListExpr below.
 	x       ast.Expr   // expression
 	lbrack  token.Pos  // position of "["
 	indices []ast.Expr // index expressions
 	rbrack  token.Pos  // position of "]"
 }

 func (x *indexedExpr) Pos() token.Pos {
 	return x.orig.Pos()
 }

 func unpackIndexedExpr(n ast.Node) *indexedExpr {
 	switch e := n.(type) {
 	case *ast.IndexExpr:
 		return &indexedExpr{
 			orig:    e,
 			x:       e.X,
 			lbrack:  e.Lbrack,
 			indices: []ast.Expr{e.Index},
 			rbrack:  e.Rbrack,
 		}
 	case *ast.IndexListExpr:
 		return &indexedExpr{
 			orig:    e,
 			x:       e.X,
 			lbrack:  e.Lbrack,
 			indices: e.Indices,
 			rbrack:  e.Rbrack,
 		}
 	}
 	return nil
 }
	// Copyright 2021 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// This file implements typechecking of index/slice expressions.

	package types

	import (
	"go/ast"
	"go/constant"
	"go/token"
	. "internal/types/errors"
	)

	// If e is a valid function instantiation, indexExpr returns true.
	// In that case x represents the uninstantiated function value and
	// it is the caller's responsibility to instantiate the function.
	func (check Checker) indexExpr(x operand, e *indexedExpr) (isFuncInst bool) {
	check.exprOrType(x, e.x, true)
	// x may be generic

	switch x.mode {
	case invalid:
	check.use(e.indices...)
	return false

	case typexpr:
	// type instantiation
	x.mode = invalid
	// TODO(gri) here we re-evaluate e.X - try to avoid this
	x.typ = check.varType(e.orig)
	if isValid(x.typ) {
	x.mode = typexpr
	}
	return false

	case value:
	if sig, _ := under(x.typ).(*Signature); sig != nil && sig.TypeParams().Len() > 0 {
	// function instantiation
	return true
	}
	}

	// x should not be generic at this point, but be safe and check
	check.nonGeneric(nil, x)
	if x.mode == invalid {
	return false
	}

	// ordinary index expression
	valid := false
	length := int64(-1) // valid if >= 0
	switch typ := under(x.typ).(type) {
	case *Basic:
	if isString(typ) {
	valid = true
	if x.mode == constant_ {
	length = int64(len(constant.StringVal(x.val)))
	}
	// an indexed string always yields a byte value
	// (not a constant) even if the string and the
	// index are constant
	x.mode = value
	x.typ = universeByte // use 'byte' name
	}

	case *Array:
	valid = true
	length = typ.len
	if x.mode != variable {
	x.mode = value
	}
	x.typ = typ.elem

	case *Pointer:
	if typ, _ := under(typ.base).(*Array); typ != nil {
	valid = true
	length = typ.len
	x.mode = variable
	x.typ = typ.elem
	}

	case *Slice:
	valid = true
	x.mode = variable
	x.typ = typ.elem

	case *Map:
	index := check.singleIndex(e)
	if index == nil {
	x.mode = invalid
	return false
	}
	var key operand
	check.expr(nil, &key, index)
	check.assignment(&key, typ.key, "map index")
	// ok to continue even if indexing failed - map element type is known
	x.mode = mapindex
	x.typ = typ.elem
	x.expr = e.orig
	return false

	case *Interface:
	if !isTypeParam(x.typ) {
	break
	}
	// TODO(gri) report detailed failure cause for better error messages
	var key, elem Type // key != nil: we must have all maps
	mode := variable // non-maps result mode
	// TODO(gri) factor out closure and use it for non-typeparam cases as well
	if underIs(x.typ, func(u Type) bool {
	l := int64(-1) // valid if >= 0
	var k, e Type // k is only set for maps
	switch t := u.(type) {
	case *Basic:
	if isString(t) {
	e = universeByte
	mode = value
	}
	case *Array:
	l = t.len
	e = t.elem
	if x.mode != variable {
	mode = value
	}
	case *Pointer:
	if t, _ := under(t.base).(*Array); t != nil {
	l = t.len
	e = t.elem
	}
	case *Slice:
	e = t.elem
	case *Map:
	k = t.key
	e = t.elem
	}
	if e == nil {
	return false
	}
	if elem == nil {
	// first type
	length = l
	key, elem = k, e
	return true
	}
	// all map keys must be identical (incl. all nil)
	// (that is, we cannot mix maps with other types)
	if !Identical(key, k) {
	return false
	}
	// all element types must be identical
	if !Identical(elem, e) {
	return false
	}
	// track the minimal length for arrays, if any
	if l >= 0 && l < length {
	length = l
	}
	return true
	}) {
	// For maps, the index expression must be assignable to the map key type.
	if key != nil {
	index := check.singleIndex(e)
	if index == nil {
	x.mode = invalid
	return false
	}
	var k operand
	check.expr(nil, &k, index)
	check.assignment(&k, key, "map index")
	// ok to continue even if indexing failed - map element type is known
	x.mode = mapindex
	x.typ = elem
	x.expr = e.orig
	return false
	}

	// no maps
	valid = true
	x.mode = mode
	x.typ = elem
	}
	}

	if !valid {
	// types2 uses the position of '[' for the error
	check.errorf(x, NonIndexableOperand, "cannot index %s", x)
	check.use(e.indices...)
	x.mode = invalid
	return false
	}

	index := check.singleIndex(e)
	if index == nil {
	x.mode = invalid
	return false
	}

	// In pathological (invalid) cases (e.g.: type T1 [][[]T1{}[0][0]]T0)
	// the element type may be accessed before it's set. Make sure we have
	// a valid type.
	if x.typ == nil {
	x.typ = Typ[Invalid]
	}

	check.index(index, length)
	return false
	}

	func (check Checker) sliceExpr(x operand, e *ast.SliceExpr) {
	check.expr(nil, x, e.X)
	if x.mode == invalid {
	check.use(e.Low, e.High, e.Max)
	return
	}

	// determine common underlying type cu
	var ct, cu Type // type and respective common underlying type
	var hasString bool
	typeset(x.typ, func(t, u Type) bool {
	if u == nil {
	check.errorf(x, NonSliceableOperand, "cannot slice %s: no specific type in %s", x, x.typ)
	cu = nil
	return false
	}

	// Treat strings like byte slices but remember that we saw a string.
	if isString(u) {
	u = NewSlice(universeByte)
	hasString = true
	}

	// If this is the first type we're seeing, we're done.
	if cu == nil {
	ct, cu = t, u
	return true
	}

	// Otherwise, the current type must have the same underlying type as all previous types.
	if !Identical(cu, u) {
	check.errorf(x, NonSliceableOperand, "cannot slice %s: %s and %s have different underlying types", x, ct, t)
	cu = nil
	return false
	}

	return true
	})
	if hasString {
	// If we saw a string, proceed with string type,
	// but don't go from untyped string to string.
	cu = Typ[String]
	if !isTypeParam(x.typ) {
	cu = under(x.typ) // untyped string remains untyped
	}
	}

	valid := false
	length := int64(-1) // valid if >= 0
	switch u := cu.(type) {
	case nil:
	// error reported above
	x.mode = invalid
	return

	case *Basic:
	if isString(u) {
	if e.Slice3 {
	at := e.Max
	if at == nil {
	at = e // e.Index[2] should be present but be careful
	}
	check.error(at, InvalidSliceExpr, invalidOp+"3-index slice of string")
	x.mode = invalid
	return
	}
	valid = true
	if x.mode == constant_ {
	length = int64(len(constant.StringVal(x.val)))
	}
	// spec: "For untyped string operands the result
	// is a non-constant value of type string."
	if isUntyped(x.typ) {
	x.typ = Typ[String]
	}
	}

	case *Array:
	valid = true
	length = u.len
	if x.mode != variable {
	check.errorf(x, NonSliceableOperand, "cannot slice unaddressable value %s", x)
	x.mode = invalid
	return
	}
	x.typ = &Slice{elem: u.elem}

	case *Pointer:
	if u, _ := under(u.base).(*Array); u != nil {
	valid = true
	length = u.len
	x.typ = &Slice{elem: u.elem}
	}

	case *Slice:
	valid = true
	// x.typ doesn't change
	}

	if !valid {
	check.errorf(x, NonSliceableOperand, "cannot slice %s", x)
	x.mode = invalid
	return
	}

	x.mode = value

	// spec: "Only the first index may be omitted; it defaults to 0."
	if e.Slice3 && (e.High == nil \|\| e.Max == nil) {
	check.error(inNode(e, e.Rbrack), InvalidSyntaxTree, "2nd and 3rd index required in 3-index slice")
	x.mode = invalid
	return
	}

	// check indices
	var ind [3]int64
	for i, expr := range []ast.Expr{e.Low, e.High, e.Max} {
	x := int64(-1)
	switch {
	case expr != nil:
	// The "capacity" is only known statically for strings, arrays,
	// and pointers to arrays, and it is the same as the length for
	// those types.
	max := int64(-1)
	if length >= 0 {
	max = length + 1
	}
	if _, v := check.index(expr, max); v >= 0 {
	x = v
	}
	case i == 0:
	// default is 0 for the first index
	x = 0
	case length >= 0:
	// default is length (== capacity) otherwise
	x = length
	}
	ind[i] = x
	}

	// constant indices must be in range
	// (check.index already checks that existing indices >= 0)
	L:
	for i, x := range ind[:len(ind)-1] {
	if x > 0 {
	for j, y := range ind[i+1:] {
	if y >= 0 && y < x {
	// The value y corresponds to the expression e.Index[i+1+j].
	// Because y >= 0, it must have been set from the expression
	// when checking indices and thus e.Index[i+1+j] is not nil.
	at := []ast.Expr{e.Low, e.High, e.Max}[i+1+j]
	check.errorf(at, SwappedSliceIndices, "invalid slice indices: %d < %d", y, x)
	break L // only report one error, ok to continue
	}
	}
	}
	}
	}

	// singleIndex returns the (single) index from the index expression e.
	// If the index is missing, or if there are multiple indices, an error
	// is reported and the result is nil.
	func (check Checker) singleIndex(expr indexedExpr) ast.Expr {
	if len(expr.indices) == 0 {
	check.errorf(expr.orig, InvalidSyntaxTree, "index expression %v with 0 indices", expr)
	return nil
	}
	if len(expr.indices) > 1 {
	// TODO(rFindley) should this get a distinct error code?
	check.error(expr.indices[1], InvalidIndex, invalidOp+"more than one index")
	}
	return expr.indices[0]
	}

	// index checks an index expression for validity.
	// If max >= 0, it is the upper bound for index.
	// If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.
	// If the result val >= 0, index is valid and val is its constant int value.
	func (check *Checker) index(index ast.Expr, max int64) (typ Type, val int64) {
	typ = Typ[Invalid]
	val = -1

	var x operand
	check.expr(nil, &x, index)
	if !check.isValidIndex(&x, InvalidIndex, "index", false) {
	return
	}

	if x.mode != constant_ {
	return x.typ, -1
	}

	if x.val.Kind() == constant.Unknown {
	return
	}

	v, ok := constant.Int64Val(x.val)
	assert(ok)
	if max >= 0 && v >= max {
	check.errorf(&x, InvalidIndex, invalidArg+"index %s out of bounds [0:%d]", x.val.String(), max)
	return
	}

	// 0 <= v [ && v < max ]
	return x.typ, v
	}

	func (check Checker) isValidIndex(x operand, code Code, what string, allowNegative bool) bool {
	if x.mode == invalid {
	return false
	}

	// spec: "a constant index that is untyped is given type int"
	check.convertUntyped(x, Typ[Int])
	if x.mode == invalid {
	return false
	}

	// spec: "the index x must be of integer type or an untyped constant"
	if !allInteger(x.typ) {
	check.errorf(x, code, invalidArg+"%s %s must be integer", what, x)
	return false
	}

	if x.mode == constant_ {
	// spec: "a constant index must be non-negative ..."
	if !allowNegative && constant.Sign(x.val) < 0 {
	check.errorf(x, code, invalidArg+"%s %s must not be negative", what, x)
	return false
	}

	// spec: "... and representable by a value of type int"
	if !representableConst(x.val, check, Typ[Int], &x.val) {
	check.errorf(x, code, invalidArg+"%s %s overflows int", what, x)
	return false
	}
	}

	return true
	}

	// indexedExpr wraps an ast.IndexExpr or ast.IndexListExpr.
	//
	// Orig holds the original ast.Expr from which this indexedExpr was derived.
	//
	// Note: indexedExpr (intentionally) does not wrap ast.Expr, as that leads to
	// accidental misuse such as encountered in golang/go#63933.
	//
	// TODO(rfindley): remove this helper, in favor of just having a helper
	// function that returns indices.
	type indexedExpr struct {
	orig ast.Expr // the wrapped expr, which may be distinct from the IndexListExpr below.
	x ast.Expr // expression
	lbrack token.Pos // position of "["
	indices []ast.Expr // index expressions
	rbrack token.Pos // position of "]"
	}

	func (x *indexedExpr) Pos() token.Pos {
	return x.orig.Pos()
	}

	func unpackIndexedExpr(n ast.Node) *indexedExpr {
	switch e := n.(type) {
	case *ast.IndexExpr:
	return &indexedExpr{
	orig: e,
	x: e.X,
	lbrack: e.Lbrack,
	indices: []ast.Expr{e.Index},
	rbrack: e.Rbrack,
	}
	case *ast.IndexListExpr:
	return &indexedExpr{
	orig: e,
	x: e.X,
	lbrack: e.Lbrack,
	indices: e.Indices,
	rbrack: e.Rbrack,
	}
	}
	return nil
	}