cmd/gc: do not omit wrapper for expression (interface{...}).F

Fixes #6723.

R=rsc
CC=golang-codereviews
https://golang.org/cl/41570047

src/cmd/gc/reflect.c

@@ -476,9 +476,8 @@ imethods(Type *t)
 			last->link = a;
 		last = a;
 
-		// Compiler can only refer to wrappers for
-		// named interface types and non-blank methods.
-		if(t->sym == S || isblanksym(method))
+		// Compiler can only refer to wrappers for non-blank methods.
+		if(isblanksym(method))
 			continue;
 
 		// NOTE(rsc): Perhaps an oversight that

test/method4.dir/prog.go

@@ -74,6 +74,13 @@ func main() {
 	eq(f4(t1, a, 17), 27)
 	eq(f4(t2, a, 18), 28)
 
+	// issue 6723
+	f5 := (interface {
+		I2
+	}).Sum
+	eq(f5(t1, a, 19), 29)
+	eq(f5(t2, a, 20), 30)
+
 	mt1 := method4a.T1(4)
 	mt2 := &method4a.T2{4}