cmd/compile/internal/types2: implement copy for generic argument types

For now, the underlying types of the the argument types' constraints
must be a single type that is a slice (the source operand may also
be a string).

Change-Id: I9e705e3349c9242f126b9c3e0af65e9ffb25fe6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/354432
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -336,15 +336,13 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 			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, "copy on generic operands not yet implemented")
 		}
 
 		if dst == nil || src == nil {

src/cmd/compile/internal/types2/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 ~[]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, x3 /* ERROR different element types */ )
+}
+
+func _[T ~[]E, E any](x T, y []E) {
+	copy(x, y)
+	copy(x /* ERROR different element types */ , "foo")
+}
+
+func _[T ~string](x []byte, y T) {
+	copy(x, y)
+	copy(y /* ERROR expects slice arguments */ , x)
+}
+
+func _[T ~[]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 }