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/16866
Reviewed-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
 }