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>

1 files changed, 4 insertions, 0 deletions

diff --git a/test/blank1.go b/test/blank1.go
index 54a72976b7..bf94d1a0fb 100644
--- a/test/blank1.go
+++ b/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
 }