go/types: factor out index/slice expr handling

This is a port of CL 308370 to go/types. There are some differences in
the index checking code, but the methodology for moving the code was the
same: replace `goto Error` with `x.mode = invalid; return`.

Change-Id: I880f577a7720e6ad8a5b096207001fcf7620396d
Reviewed-on: https://go-review.googlesource.com/c/go/+/312095
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

2 files changed, 412 insertions, 378 deletions

diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 7d701d985b..bdab7d9aa6 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -1002,121 +1002,6 @@ func (check *Checker) binary(x *operand, e ast.Expr, lhs, rhs ast.Expr, op token
 	// x.typ is unchanged
 }
 
-// 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(&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.invalidArg(&x, _InvalidIndex, "index %s is out of bounds", &x)
-		return
-	}
-
-	// 0 <= v [ && v < max ]
-	return x.typ, v
-}
-
-func (check *Checker) isValidIndex(x *operand, code errorCode, 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 !isInteger(x.typ) {
-		check.invalidArg(x, code, "%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.invalidArg(x, code, "%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.invalidArg(x, code, "%s %s overflows int", what, x)
-			return false
-		}
-	}
-
-	return true
-}
-
-// indexElts checks the elements (elts) of an array or slice composite literal
-// against the literal's element type (typ), and the element indices against
-// the literal length if known (length >= 0). It returns the length of the
-// literal (maximum index value + 1).
-//
-func (check *Checker) indexedElts(elts []ast.Expr, typ Type, length int64) int64 {
-	visited := make(map[int64]bool, len(elts))
-	var index, max int64
-	for _, e := range elts {
-		// determine and check index
-		validIndex := false
-		eval := e
-		if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
-			if typ, i := check.index(kv.Key, length); typ != Typ[Invalid] {
-				if i >= 0 {
-					index = i
-					validIndex = true
-				} else {
-					check.errorf(e, _InvalidLitIndex, "index %s must be integer constant", kv.Key)
-				}
-			}
-			eval = kv.Value
-		} else if length >= 0 && index >= length {
-			check.errorf(e, _OversizeArrayLit, "index %d is out of bounds (>= %d)", index, length)
-		} else {
-			validIndex = true
-		}
-
-		// if we have a valid index, check for duplicate entries
-		if validIndex {
-			if visited[index] {
-				check.errorf(e, _DuplicateLitKey, "duplicate index %d in array or slice literal", index)
-			}
-			visited[index] = true
-		}
-		index++
-		if index > max {
-			max = index
-		}
-
-		// check element against composite literal element type
-		var x operand
-		check.exprWithHint(&x, eval, typ)
-		check.assignment(&x, typ, "array or slice literal")
-	}
-	return max
-}
-
 // exprKind describes the kind of an expression; the kind
 // determines if an expression is valid in 'statement context'.
 type exprKind int
@@ -1455,278 +1340,17 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		check.selector(x, e)
 
 	case *ast.IndexExpr:
-		check.exprOrType(x, e.X)
+		check.indexExpr(x, e)
 		if x.mode == invalid {
-			check.use(typeparams.UnpackExpr(e.Index)...)
-			goto Error
-		}
-
-		if x.mode == typexpr {
-			// type instantiation
-			x.mode = invalid
-			x.typ = check.varType(e)
-			if x.typ != Typ[Invalid] {
-				x.mode = typexpr
-			}
-			return expression
-		}
-
-		if x.mode == value {
-			if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
-				check.funcInst(x, e)
-				return expression
-			}
-		}
-
-		valid := false
-		length := int64(-1) // valid if >= 0
-		switch typ := optype(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 := asArray(typ.base); 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:
-			var key operand
-			check.expr(&key, e.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
-			return expression
-
-		case *_Sum:
-			// A sum type can be indexed if all of the sum's types
-			// support indexing and have the same index and element
-			// type. Special rules apply for maps in the sum type.
-			var tkey, telem Type // key is for map types only
-			nmaps := 0           // number of map types in sum type
-			if typ.is(func(t Type) bool {
-				var e Type
-				switch t := under(t).(type) {
-				case *Basic:
-					if isString(t) {
-						e = universeByte
-					}
-				case *Array:
-					e = t.elem
-				case *Pointer:
-					if t := asArray(t.base); t != nil {
-						e = t.elem
-					}
-				case *Slice:
-					e = t.elem
-				case *Map:
-					// If there are multiple maps in the sum type,
-					// they must have identical key types.
-					// TODO(gri) We may be able to relax this rule
-					// but it becomes complicated very quickly.
-					if tkey != nil && !Identical(t.key, tkey) {
-						return false
-					}
-					tkey = t.key
-					e = t.elem
-					nmaps++
-				case *_TypeParam:
-					check.errorf(x, 0, "type of %s contains a type parameter - cannot index (implementation restriction)", x)
-				case *instance:
-					panic("unimplemented")
-				}
-				if e == nil || telem != nil && !Identical(e, telem) {
-					return false
-				}
-				telem = e
-				return true
-			}) {
-				// If there are maps, the index expression must be assignable
-				// to the map key type (as for simple map index expressions).
-				if nmaps > 0 {
-					var key operand
-					check.expr(&key, e.Index)
-					check.assignment(&key, tkey, "map index")
-					// ok to continue even if indexing failed - map element type is known
-
-					// If there are only maps, we are done.
-					if nmaps == len(typ.types) {
-						x.mode = mapindex
-						x.typ = telem
-						x.expr = e
-						return expression
-					}
-
-					// Otherwise we have mix of maps and other types. For
-					// now we require that the map key be an integer type.
-					// TODO(gri) This is probably not good enough.
-					valid = isInteger(tkey)
-					// avoid 2nd indexing error if indexing failed above
-					if !valid && key.mode == invalid {
-						goto Error
-					}
-					x.mode = value // map index expressions are not addressable
-				} else {
-					// no maps
-					valid = true
-					x.mode = variable
-				}
-				x.typ = telem
-			}
-		}
-
-		if !valid {
-			check.invalidOp(x, _NonIndexableOperand, "cannot index %s", x)
-			goto Error
-		}
-
-		if e.Index == nil {
-			check.invalidAST(e, "missing index for %s", x)
 			goto Error
 		}
 
-		// 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(e.Index, length)
-		// ok to continue
-
 	case *ast.SliceExpr:
-		check.expr(x, e.X)
+		check.sliceExpr(x, e)
 		if x.mode == invalid {
-			check.use(e.Low, e.High, e.Max)
 			goto Error
 		}
 
-		valid := false
-		length := int64(-1) // valid if >= 0
-		switch typ := optype(x.typ).(type) {
-		case *Basic:
-			if isString(typ) {
-				if e.Slice3 {
-					check.invalidOp(x, _InvalidSliceExpr, "3-index slice of string")
-					goto Error
-				}
-				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 typ.kind == UntypedString {
-					x.typ = Typ[String]
-				}
-			}
-
-		case *Array:
-			valid = true
-			length = typ.len
-			if x.mode != variable {
-				check.invalidOp(x, _NonSliceableOperand, "cannot slice %s (value not addressable)", x)
-				goto Error
-			}
-			x.typ = &Slice{elem: typ.elem}
-
-		case *Pointer:
-			if typ := asArray(typ.base); typ != nil {
-				valid = true
-				length = typ.len
-				x.typ = &Slice{elem: typ.elem}
-			}
-
-		case *Slice:
-			valid = true
-			// x.typ doesn't change
-
-		case *_Sum, *_TypeParam:
-			check.errorf(x, 0, "generic slice expressions not yet implemented")
-			goto Error
-		}
-
-		if !valid {
-			check.invalidOp(x, _NonSliceableOperand, "cannot slice %s", x)
-			goto Error
-		}
-
-		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.invalidAST(inNode(e, e.Rbrack), "2nd and 3rd index required in 3-index slice")
-			goto Error
-		}
-
-		// 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 _, y := range ind[i+1:] {
-					if y >= 0 && x > y {
-						check.errorf(inNode(e, e.Rbrack), _SwappedSliceIndices, "swapped slice indices: %d > %d", x, y)
-						break L // only report one error, ok to continue
-					}
-				}
-			}
-		}
-
 	case *ast.TypeAssertExpr:
 		check.expr(x, e.X)
 		if x.mode == invalid {
diff --git a/src/go/types/index.go b/src/go/types/index.go
new file mode 100644
index 0000000000..f497b06dad
--- /dev/null
+++ b/src/go/types/index.go
@@ -0,0 +1,410 @@
+// 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/internal/typeparams"
+)
+
+func (check *Checker) indexExpr(x *operand, e *ast.IndexExpr) {
+	check.exprOrType(x, e.X)
+	if x.mode == invalid {
+		check.use(typeparams.UnpackExpr(e.Index)...)
+		return
+	}
+
+	if x.mode == typexpr {
+		// type instantiation
+		x.mode = invalid
+		x.typ = check.varType(e)
+		if x.typ != Typ[Invalid] {
+			x.mode = typexpr
+		}
+		return
+	}
+
+	if x.mode == value {
+		if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
+			check.funcInst(x, e)
+			return
+		}
+	}
+
+	valid := false
+	length := int64(-1) // valid if >= 0
+	switch typ := optype(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 := asArray(typ.base); 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:
+		var key operand
+		check.expr(&key, e.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
+		return
+
+	case *_Sum:
+		// A sum type can be indexed if all of the sum's types
+		// support indexing and have the same index and element
+		// type. Special rules apply for maps in the sum type.
+		var tkey, telem Type // key is for map types only
+		nmaps := 0           // number of map types in sum type
+		if typ.is(func(t Type) bool {
+			var e Type
+			switch t := under(t).(type) {
+			case *Basic:
+				if isString(t) {
+					e = universeByte
+				}
+			case *Array:
+				e = t.elem
+			case *Pointer:
+				if t := asArray(t.base); t != nil {
+					e = t.elem
+				}
+			case *Slice:
+				e = t.elem
+			case *Map:
+				// If there are multiple maps in the sum type,
+				// they must have identical key types.
+				// TODO(gri) We may be able to relax this rule
+				// but it becomes complicated very quickly.
+				if tkey != nil && !Identical(t.key, tkey) {
+					return false
+				}
+				tkey = t.key
+				e = t.elem
+				nmaps++
+			case *_TypeParam:
+				check.errorf(x, 0, "type of %s contains a type parameter - cannot index (implementation restriction)", x)
+			case *instance:
+				panic("unimplemented")
+			}
+			if e == nil || telem != nil && !Identical(e, telem) {
+				return false
+			}
+			telem = e
+			return true
+		}) {
+			// If there are maps, the index expression must be assignable
+			// to the map key type (as for simple map index expressions).
+			if nmaps > 0 {
+				var key operand
+				check.expr(&key, e.Index)
+				check.assignment(&key, tkey, "map index")
+				// ok to continue even if indexing failed - map element type is known
+
+				// If there are only maps, we are done.
+				if nmaps == len(typ.types) {
+					x.mode = mapindex
+					x.typ = telem
+					x.expr = e
+					return
+				}
+
+				// Otherwise we have mix of maps and other types. For
+				// now we require that the map key be an integer type.
+				// TODO(gri) This is probably not good enough.
+				valid = isInteger(tkey)
+				// avoid 2nd indexing error if indexing failed above
+				if !valid && key.mode == invalid {
+					x.mode = invalid
+					return
+				}
+				x.mode = value // map index expressions are not addressable
+			} else {
+				// no maps
+				valid = true
+				x.mode = variable
+			}
+			x.typ = telem
+		}
+	}
+
+	if !valid {
+		check.invalidOp(x, _NonIndexableOperand, "cannot index %s", x)
+		x.mode = invalid
+		return
+	}
+
+	if e.Index == nil {
+		check.invalidAST(e, "missing index for %s", x)
+		x.mode = invalid
+		return
+	}
+
+	// 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(e.Index, length)
+}
+
+func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
+	check.expr(x, e.X)
+	if x.mode == invalid {
+		check.use(e.Low, e.High, e.Max)
+		return
+	}
+
+	valid := false
+	length := int64(-1) // valid if >= 0
+	switch typ := optype(x.typ).(type) {
+	case *Basic:
+		if isString(typ) {
+			if e.Slice3 {
+				check.invalidOp(x, _InvalidSliceExpr, "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 typ.kind == UntypedString {
+				x.typ = Typ[String]
+			}
+		}
+
+	case *Array:
+		valid = true
+		length = typ.len
+		if x.mode != variable {
+			check.invalidOp(x, _NonSliceableOperand, "cannot slice %s (value not addressable)", x)
+			x.mode = invalid
+			return
+		}
+		x.typ = &Slice{elem: typ.elem}
+
+	case *Pointer:
+		if typ := asArray(typ.base); typ != nil {
+			valid = true
+			length = typ.len
+			x.typ = &Slice{elem: typ.elem}
+		}
+
+	case *Slice:
+		valid = true
+		// x.typ doesn't change
+
+	case *_Sum, *_TypeParam:
+		check.errorf(x, 0, "generic slice expressions not yet implemented")
+		x.mode = invalid
+		return
+	}
+
+	if !valid {
+		check.invalidOp(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.invalidAST(inNode(e, e.Rbrack), "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 _, y := range ind[i+1:] {
+				if y >= 0 && x > y {
+					check.errorf(inNode(e, e.Rbrack), _SwappedSliceIndices, "swapped slice indices: %d > %d", x, y)
+					break L // only report one error, ok to continue
+				}
+			}
+		}
+	}
+}
+
+// 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(&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.invalidArg(&x, _InvalidIndex, "index %s is out of bounds", &x)
+		return
+	}
+
+	// 0 <= v [ && v < max ]
+	return x.typ, v
+}
+
+func (check *Checker) isValidIndex(x *operand, code errorCode, 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 !isInteger(x.typ) {
+		check.invalidArg(x, code, "%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.invalidArg(x, code, "%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.invalidArg(x, code, "%s %s overflows int", what, x)
+			return false
+		}
+	}
+
+	return true
+}
+
+// indexElts checks the elements (elts) of an array or slice composite literal
+// against the literal's element type (typ), and the element indices against
+// the literal length if known (length >= 0). It returns the length of the
+// literal (maximum index value + 1).
+//
+func (check *Checker) indexedElts(elts []ast.Expr, typ Type, length int64) int64 {
+	visited := make(map[int64]bool, len(elts))
+	var index, max int64
+	for _, e := range elts {
+		// determine and check index
+		validIndex := false
+		eval := e
+		if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
+			if typ, i := check.index(kv.Key, length); typ != Typ[Invalid] {
+				if i >= 0 {
+					index = i
+					validIndex = true
+				} else {
+					check.errorf(e, _InvalidLitIndex, "index %s must be integer constant", kv.Key)
+				}
+			}
+			eval = kv.Value
+		} else if length >= 0 && index >= length {
+			check.errorf(e, _OversizeArrayLit, "index %d is out of bounds (>= %d)", index, length)
+		} else {
+			validIndex = true
+		}
+
+		// if we have a valid index, check for duplicate entries
+		if validIndex {
+			if visited[index] {
+				check.errorf(e, _DuplicateLitKey, "duplicate index %d in array or slice literal", index)
+			}
+			visited[index] = true
+		}
+		index++
+		if index > max {
+			max = index
+		}
+
+		// check element against composite literal element type
+		var x operand
+		check.exprWithHint(&x, eval, typ)
+		check.assignment(&x, typ, "array or slice literal")
+	}
+	return max
+}