go/types, types2: implement type checking of "clear" built-in

Will become available with Go 1.21.

Recognizing the `clear` built-in early is not causing any problems:
if existing code defines a `clear`, that will be used as before. If
code doesn't define `clear` the error message will make it clear
that with 1.21 the function will be available. It's still possible
to define a local `clear` and get rid of the error; but more likely
the name choice should be avoided going forward, so this provides a
useful early "heads-up".

For #56351.

Change-Id: I3d0fb1eb3508fbc78d7514b6238eac89610158c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/448076
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/types2/builtins.go

@@ -232,6 +232,33 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 		x.typ = Typ[Int]
 		x.val = val
 
+	case _Clear:
+		// clear(m)
+		if !check.allowVersion(check.pkg, 1, 21) {
+			check.versionErrorf(call.Fun, "go1.21", "clear")
+			return
+		}
+
+		if !underIs(x.typ, func(u Type) bool {
+			switch u := u.(type) {
+			case *Map, *Slice:
+				return true
+			case *Pointer:
+				if _, ok := under(u.base).(*Array); ok {
+					return true
+				}
+			}
+			check.errorf(x, InvalidClear, invalidArg+"cannot clear %s: argument must be (or constrained by) map, slice, or array pointer", x)
+			return false
+		}) {
+			return
+		}
+
+		x.mode = novalue
+		if check.recordTypes() {
+			check.recordBuiltinType(call.Fun, makeSig(nil, x.typ))
+		}
+
 	case _Close:
 		// close(c)
 		if !underIs(x.typ, func(u Type) bool {

src/cmd/compile/internal/types2/builtins_test.go

@@ -41,6 +41,11 @@ var builtinCalls = []struct {
 	{"len", `type S []byte; var s S; _ = len(s)`, `func(p.S) int`},
 	{"len", `var s P; _ = len(s)`, `func(P) int`},
 
+	{"clear", `var m map[float64]int; clear(m)`, `func(map[float64]int)`},
+	{"clear", `var s []byte; clear(s)`, `func([]byte)`},
+	{"clear", `var p *[10]int; clear(p)`, `func(*[10]int)`},
+	{"clear", `var s P; clear(s)`, `func(P)`},
+
 	{"close", `var c chan int; close(c)`, `func(chan int)`},
 	{"close", `var c chan<- chan string; close(c)`, `func(chan<- chan string)`},
 

src/cmd/compile/internal/types2/universe.go

@@ -145,6 +145,7 @@ const (
 	// universe scope
 	_Append builtinId = iota
 	_Cap
+	_Clear
 	_Close
 	_Complex
 	_Copy
@@ -182,6 +183,7 @@ var predeclaredFuncs = [...]struct {
 }{
 	_Append:  {"append", 1, true, expression},
 	_Cap:     {"cap", 1, false, expression},
+	_Clear:   {"clear", 1, false, statement},
 	_Close:   {"close", 1, false, statement},
 	_Complex: {"complex", 2, false, expression},
 	_Copy:    {"copy", 2, false, statement},

src/go/types/builtins.go

@@ -233,6 +233,33 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		x.typ = Typ[Int]
 		x.val = val
 
+	case _Clear:
+		// clear(m)
+		if !check.allowVersion(check.pkg, 1, 21) {
+			check.error(call.Fun, UnsupportedFeature, "clear requires go1.21 or later")
+			return
+		}
+
+		if !underIs(x.typ, func(u Type) bool {
+			switch u := u.(type) {
+			case *Map, *Slice:
+				return true
+			case *Pointer:
+				if _, ok := under(u.base).(*Array); ok {
+					return true
+				}
+			}
+			check.errorf(x, InvalidClear, invalidArg+"cannot clear %s: argument must be (or constrained by) map, slice, or array pointer", x)
+			return false
+		}) {
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, x.typ))
+		}
+
 	case _Close:
 		// close(c)
 		if !underIs(x.typ, func(u Type) bool {

src/go/types/builtins_test.go

@@ -42,6 +42,11 @@ var builtinCalls = []struct {
 	{"len", `type S []byte; var s S; _ = len(s)`, `func(p.S) int`},
 	{"len", `var s P; _ = len(s)`, `func(P) int`},
 
+	{"clear", `var m map[float64]int; clear(m)`, `func(map[float64]int)`},
+	{"clear", `var s []byte; clear(s)`, `func([]byte)`},
+	{"clear", `var p *[10]int; clear(p)`, `func(*[10]int)`},
+	{"clear", `var s P; clear(s)`, `func(P)`},
+
 	{"close", `var c chan int; close(c)`, `func(chan int)`},
 	{"close", `var c chan<- chan string; close(c)`, `func(chan<- chan string)`},
 

src/go/types/universe.go

@@ -146,6 +146,7 @@ const (
 	// universe scope
 	_Append builtinId = iota
 	_Cap
+	_Clear
 	_Close
 	_Complex
 	_Copy
@@ -183,6 +184,7 @@ var predeclaredFuncs = [...]struct {
 }{
 	_Append:  {"append", 1, true, expression},
 	_Cap:     {"cap", 1, false, expression},
+	_Clear:   {"clear", 1, false, statement},
 	_Close:   {"close", 1, false, statement},
 	_Complex: {"complex", 2, false, expression},
 	_Copy:    {"copy", 2, false, statement},

src/internal/types/errors/codes.go

@@ -1430,4 +1430,13 @@ const (
 	// InvalidUnsafeStringData occurs if it is used in a package
 	// compiled for a language version before go1.20.
 	_ // not used anymore
+
+	// InvalidClear occurs when clear is called with an argument
+	// that is not of map, slice, or pointer-to-array type.
+	//
+	// Example:
+	//  func _(x int) {
+	//  	clear(x)
+	//  }
+	InvalidClear
 )

src/internal/types/testdata/check/builtins0.go

@@ -139,6 +139,17 @@ func cap3() {
 	)
 }
 
+func clear1() {
+	var a [10]int
+	var m map[float64]string
+	var s []byte
+	clear(a /* ERROR cannot clear a */)
+	clear(&a)
+	clear(m)
+	clear(s)
+	clear([]int{})
+}
+
 func close1() {
 	var c chan int
 	var r <-chan int

src/internal/types/testdata/check/builtins1.go

@@ -8,6 +8,20 @@ package builtins
 
 import "unsafe"
 
+// clear
+
+func _[T any](x T) {
+	clear(x /* ERROR cannot clear x */)
+}
+
+func _[T ~map[int]string | ~[]byte | ~*[10]int](x T) {
+	clear(x)
+}
+
+func _[T ~map[int]string | ~[]byte | ~*[10]int | string](x T) {
+	clear(x /* ERROR cannot clear x */)
+}
+
 // close
 
 type C0 interface{ int }

src/internal/types/testdata/fixedbugs/issue56351.go

@@ -0,0 +1,11 @@
+// -lang=go1.20
+
+// Copyright 2022 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.
+
+package p
+
+func _(s []int) {
+	clear /* ERROR clear requires go1\.21 or later */ (s)
+}