cmd/compile: don't allow blank method declarations on builtins

Move test for isblank into addmethod so that most of the type checking
for methods is also performed for blank methods.

Fixes #11366.

Change-Id: I13d554723bf96d906d0b3ff390d7b7c87c1a5020
Reviewed-on: https://go-review.googlesource.com/16866Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -1374,6 +1374,15 @@ func addmethod(sf *Sym, t *Type, local bool, nointerface bool) {
 	}
 
 	pa = f
+	if local && !pa.Local {
+		Yyerror("cannot define new methods on non-local type %v", pa)
+		return
+	}
+
+	if isblanksym(sf) {
+		return
+	}
+
 	if pa.Etype == TSTRUCT {
 		for f := pa.Type; f != nil; f = f.Down {
 			if f.Sym == sf {
@@ -1383,13 +1392,6 @@ func addmethod(sf *Sym, t *Type, local bool, nointerface bool) {
 		}
 	}
 
-	if local && !pa.Local {
-		// defining method on non-local type.
-		Yyerror("cannot define new methods on non-local type %v", pa)
-
-		return
-	}
-
 	n := Nod(ODCLFIELD, newname(sf), nil)
 	n.Type = t
 

src/cmd/compile/internal/gc/typecheck.go

@@ -3441,7 +3441,7 @@ func typecheckfunc(n *Node) {
 	n.Type = t
 	t.Nname = n.Func.Nname
 	rcvr := getthisx(t).Type
-	if rcvr != nil && n.Func.Shortname != nil && !isblank(n.Func.Shortname) {
+	if rcvr != nil && n.Func.Shortname != nil {
 		addmethod(n.Func.Shortname.Sym, t, true, n.Func.Nname.Nointerface)
 	}
 

test/blank1.go

@@ -13,6 +13,10 @@ var t struct {
 	_ int
 }
 
+func (x int) _() { // ERROR "cannot define new methods on non-local type"
+	println(x)
+}
+
 type T struct {
       _ []int
 }