cmd/gc: don't attempt to generate wrappers for blank interface methods

Fixes #5691.

R=golang-dev, bradfitz, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/10255047

src/cmd/gc/reflect.c

@@ -265,8 +265,8 @@ imethods(Type *t)
 		last = a;
 
 		// Compiler can only refer to wrappers for
-		// named interface types.
-		if(t->sym == S)
+		// named interface types and non-blank methods.
+		if(t->sym == S || isblanksym(method))
 			continue;
 
 		// NOTE(rsc): Perhaps an oversight that

test/interface/explicit.go

@@ -80,3 +80,22 @@ var m2 M = jj // ERROR "incompatible|wrong type for M method"
 
 var m3 = M(ii) // ERROR "invalid|missing"
 var m4 = M(jj) // ERROR "invalid|wrong type for M method"
+
+
+type B1 interface {
+	_()
+}
+
+type B2 interface {
+	M()
+	_()
+}
+
+type T2 struct{}
+
+func (t *T2) M() {}
+func (t *T2) _() {}
+
+// Check that nothing satisfies an interface with blank methods.
+var b1 B1 = &T2{} // ERROR "incompatible|missing _ method"
+var b2 B2 = &T2{} // ERROR "incompatible|missing _ method"

test/interface/fail.go

@@ -14,18 +14,33 @@ type I interface {
 
 func main() {
 	shouldPanic(p1)
+	shouldPanic(p2)
 }
 
 func p1() {
 	var s *S
 	var i I
-	var e interface {}
+	var e interface{}
 	e = s
 	i = e.(I)
 	_ = i
 }
 
-type S struct {
+type S struct{}
+
+func (s *S) _() {}
+
+type B interface {
+	_()
+}
+
+func p2() {
+	var s *S
+	var b B
+	var e interface{}
+	e = s
+	b = e.(B)
+	_ = b
 }
 
 func shouldPanic(f func()) {