[dev.typeparams] go/types: make Interface.obj a *TypeName

This is a straightforward port of CL 332011 to go/types.

Change-Id: I682791886c8496c52094f3688e36934afbd7a241
Reviewed-on: https://go-review.googlesource.com/c/go/+/335035
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>

src/go/types/call.go

@@ -482,11 +482,10 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 			var why string
 			if tpar := asTypeParam(x.typ); tpar != nil {
 				// Type parameter bounds don't specify fields, so don't mention "field".
-				switch obj := tpar.Bound().obj.(type) {
-				case nil:
+				if tname := tpar.Bound().obj; tname != nil {
+					why = check.sprintf("interface %s has no method %s", tname.name, sel)
+				} else {
 					why = check.sprintf("type bound for %s has no method %s", x.typ, sel)
-				case *TypeName:
-					why = check.sprintf("interface %s has no method %s", obj.name, sel)
 				}
 			} else {
 				why = check.sprintf("type %s has no field or method %s", x.typ, sel)

src/go/types/sizeof_test.go

@@ -27,7 +27,7 @@ func TestSizeof(t *testing.T) {
 		{Tuple{}, 12, 24},
 		{Signature{}, 44, 88},
 		{Union{}, 24, 48},
-		{Interface{}, 44, 88},
+		{Interface{}, 40, 80},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
 		{Named{}, 84, 160},

src/go/types/type.go

@@ -258,7 +258,7 @@ func (s *Signature) Variadic() bool { return s.variadic }
 
 // An Interface represents an interface type.
 type Interface struct {
-	obj       Object       // type name object defining this interface; or nil (for better error messages)
+	obj       *TypeName    // type name object defining this interface; or nil (for better error messages)
 	methods   []*Func      // ordered list of explicitly declared methods
 	embeddeds []Type       // ordered list of explicitly embedded elements
 	embedPos  *[]token.Pos // positions of embedded elements; or nil (for error messages) - use pointer to save space