fix for subscript assignemnt with no args

tests/test_parser.py

@@ -831,6 +831,9 @@ def test_subscript_assginment(self, space):
         assert space.parse("x[0] = 5") == ast.Main(ast.Block([
             ast.Statement(ast.Assignment(ast.Subscript(ast.Send(ast.Self(1), "x", [], None, 1), [ast.ConstantInt(0)], 1), ast.ConstantInt(5)))
         ]))
+        assert space.parse("x[] = 5") == ast.Main(ast.Block([
+            ast.Statement(ast.Assignment(ast.Subscript(ast.Send(ast.Self(1), "x", [], None, 1), [], 1), ast.ConstantInt(5)))
+        ]))
 
     def test_def(self, space):
         assert space.parse("def f() end") == ast.Main(ast.Block([

topaz/parser.py

@@ -951,7 +951,8 @@ def lhs_variable(self, p):
 
     @pg.production("lhs : primary_value LITERAL_LBRACKET opt_call_args rbracket")
     def lhs_subscript(self, p):
-        return BoxAST(ast.Subscript(p[0].getast(), p[2].getcallargs(), p[1].getsourcepos().lineno))
+        args = p[2].getcallargs() if p[2] is not None else []
+        return BoxAST(ast.Subscript(p[0].getast(), args, p[1].getsourcepos().lineno))
 
     @pg.production("lhs : primary_value DOT IDENTIFIER")
     def lhs_dot_identifier(self, p):