compiler: better error messages for missing interface method

For golang/go#10700

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273886

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-534fb907c821b052dc430330708d7fa555b91fe3
+68b1c7659a6b25d537a4ff3365ab070fa6215b0b
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -12052,6 +12052,15 @@ Type::bind_field_or_method(Gogo* gogo, const Type* type, Expression* expr,
 		    ambig2.c_str());
       else if (found_pointer_method)
 	go_error_at(location, "method requires a pointer receiver");
+      else if (it != NULL && it->is_empty())
+	go_error_at(location,
+		    "reference to method %qs in interface with no methods",
+		    Gogo::message_name(name).c_str());
+      else if (it == NULL && type->deref()->interface_type() != NULL)
+	go_error_at(location,
+		    ("reference to method %qs in type that is "
+		     "pointer to interface, not interface"),
+		    Gogo::message_name(name).c_str());
       else if (nt == NULL && st == NULL && it == NULL)
 	go_error_at(location,
 		    ("reference to field %qs in object which "

gcc/testsuite/go.test/test/fixedbugs/bug340.go

@@ -1,6 +1,6 @@
 // errorcheck
 
-// Copyright 2011 The Go Authors.  All rights reserved.
+// Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
@@ -12,6 +12,6 @@ func main() {
 	var x interface{}
 	switch t := x.(type) {
 	case 0:		// ERROR "type"
-		t.x = 1 // ERROR "type interface \{\}|reference to undefined field or method"
+		t.x = 1 // ERROR "type interface \{\}|reference to undefined field or method|interface with no methods"
 	}
 }