cmd/compile/internal/types2: factor out slice elem computation for copy built-in

Implement singleUnder[String] which determines a single underlying type
for a given type: either the underlying type, or the single underlying
type for a type parameter, if it exists. Use singleUnder[String] instead
of optype for copy built-in.

This CL removes a dependency on optype and also makes the copy built-in
slighty more general for generic arguments (the source argument may be
constrained by a slice or string simultaneously).

Change-Id: Ia329e96afc69a09d2ca3b1f82fe712d4f7ba1d9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/357413
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -325,33 +325,22 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 
 	case _Copy:
 		// copy(x, y []T) int
-		var dst Type
-		if t := asSlice(x.typ); t != nil {
-			dst = t.elem
-		}
+		dst, _ := singleUnder(x.typ).(*Slice)
 
 		var y operand
 		arg(&y, 1)
 		if y.mode == invalid {
 			return
 		}
-		var src Type
-		switch t := optype(y.typ).(type) {
-		case *Basic:
-			if isString(y.typ) {
-				src = universeByte
-			}
-		case *Slice:
-			src = t.elem
-		}
+		src, _ := singleUnderString(y.typ).(*Slice)
 
 		if dst == nil || src == nil {
 			check.errorf(x, invalidArg+"copy expects slice arguments; found %s and %s", x, &y)
 			return
 		}
 
-		if !Identical(dst, src) {
-			check.errorf(x, invalidArg+"arguments to copy %s and %s have different element types %s and %s", x, &y, dst, src)
+		if !Identical(dst.elem, src.elem) {
+			check.errorf(x, invalidArg+"arguments to copy %s and %s have different element types %s and %s", x, &y, dst.elem, src.elem)
 			return
 		}
 
@@ -774,6 +763,46 @@ func (check *Checker) builtin(x *operand, call *syntax.CallExpr, id builtinId) (
 	return true
 }
 
+// If typ is a type parameter, single under returns the single underlying
+// type of all types in the corresponding type constraint if it exists, or
+// nil if it doesn't exist. If typ is not a type parameter, singleUnder
+// just returns the underlying type.
+func singleUnder(typ Type) Type {
+	var su Type
+	if underIs(typ, func(u Type) bool {
+		if su != nil && !Identical(su, u) {
+			return false
+		}
+		// su == nil || Identical(su, u)
+		su = u
+		return true
+	}) {
+		return su
+	}
+	return nil
+}
+
+// singleUnderString is like singleUnder but also considers []byte and
+// string as "identical". In this case, if successful, the result is always
+// []byte.
+func singleUnderString(typ Type) Type {
+	var su Type
+	if underIs(typ, func(u Type) bool {
+		if isString(u) {
+			u = NewSlice(universeByte)
+		}
+		if su != nil && !Identical(su, u) {
+			return false
+		}
+		// su == nil || Identical(su, u)
+		su = u
+		return true
+	}) {
+		return su
+	}
+	return nil
+}
+
 // hasVarSize reports if the size of type t is variable due to type parameters.
 func hasVarSize(t Type) bool {
 	switch t := under(t).(type) {

src/cmd/compile/internal/types2/testdata/check/builtins.go2

@@ -78,8 +78,14 @@ func _[T ~string](x []byte, y T) {
 
 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)
+	copy(y, x)
+}
+
+type L0 []int
+type L1 []int
+
+func _[T L0 | L1](x, y T) {
+	copy(x, y)
 }
 
 // delete