disallow ordinary-type.(T), as in spec.

R=ken
OCL=25705
CL=25705

src/cmd/gc/walk.c

@@ -2851,6 +2851,9 @@ ifaceas1(Type *dst, Type *src, int explicit)
 	if(src == T || dst == T)
 		return Inone;
 
+	if(explicit && !isinter(src))
+		yyerror("cannot use .(T) on non-interface type %T", src);
+
 	if(isinter(dst)) {
 		if(isinter(src)) {
 			if(eqtype(dst, src, 0))

src/lib/reflect/type.go

@@ -107,7 +107,8 @@ func newBasicType(name string, kind int, size int) Type {
 // Prebuilt basic types
 var (
 	Missing = newBasicType(missingString, MissingKind, 1);
-	DotDotDot = newBasicType(dotDotDotString, DotDotDotKind, unsafe.Sizeof(true.(interface{})));
+	empty interface{};
+	DotDotDot = newBasicType(dotDotDotString, DotDotDotKind, unsafe.Sizeof(empty));
 	Bool = newBasicType("bool", BoolKind, unsafe.Sizeof(true));
 	Int = newBasicType("int", IntKind, unsafe.Sizeof(int(0)));
 	Int8 = newBasicType("int8", Int8Kind, 1);