go/types: implement copy for generic argument types

This is a port of CL 354432 from types2 to go/types
with minor adjustments:
- an error message has a different position
- the constraint literals are wrapped in interfaces
  because the interface-free notation has not been
  ported yet

Change-Id: I167094b57b39027566f2b7ce3aa97a071bae4da5
Reviewed-on: https://go-review.googlesource.com/c/go/+/354489
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 38 insertions, 3 deletions

diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
index d805e46666..3e2c994b09 100644
--- a/src/go/types/builtins.go
+++ b/src/go/types/builtins.go
@@ -341,15 +341,13 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			return
 		}
 		var src Type
-		switch t := under(y.typ).(type) {
+		switch t := optype(y.typ).(type) {
 		case *Basic:
 			if isString(y.typ) {
 				src = universeByte
 			}
 		case *Slice:
 			src = t.elem
-		case *TypeParam:
-			check.error(x, _Todo, "copy on generic operands not yet implemented")
 		}
 
 		if dst == nil || src == nil {
diff --git a/src/go/types/testdata/check/builtins.go2 b/src/go/types/testdata/check/builtins.go2
index 1c773cc70b..fb912a1918 100644
--- a/src/go/types/testdata/check/builtins.go2
+++ b/src/go/types/testdata/check/builtins.go2
@@ -45,6 +45,43 @@ func _[T C5[X], X any](ch T) {
 	close(ch)
 }
 
+// copy
+
+func _[T any](x, y T) {
+	copy(x /* ERROR copy expects slice arguments */ , y)
+}
+
+func _[T interface{~[]byte}](x, y T) {
+	copy(x, y)
+	copy(x, "foo")
+	copy("foo" /* ERROR expects slice arguments */ , y)
+
+	var x2 []byte
+	copy(x2, y) // element types are identical
+	copy(y, x2) // element types are identical
+
+	type myByte byte
+	var x3 []myByte
+	copy(x3 /* ERROR different element types */ , y)
+	copy(y /* ERROR different element types */ , x3)
+}
+
+func _[T interface{~[]E}, E any](x T, y []E) {
+	copy(x, y)
+	copy(x /* ERROR different element types */ , "foo")
+}
+
+func _[T interface{~string}](x []byte, y T) {
+	copy(x, y)
+	copy(y /* ERROR expects slice arguments */ , x)
+}
+
+func _[T interface{~[]byte|~string}](x T, y []byte) {
+	copy(x /* ERROR expects slice arguments */ , y)
+	// TODO(gri) should this be valid?
+	copy(y /* ERROR expects slice arguments */ , x)
+}
+
 // delete
 
 type M0 interface{ int }