Merge pull request #1198 from lark-parser/issue1173

Fix for issue #1173

Author: erezsh

lark/load_grammar.py

@@ -1050,6 +1050,8 @@ def _make_rule_tuple(modifiers_tree, name, params, priority_tree, expansions):
     if modifiers_tree.children:
         m ,= modifiers_tree.children
         expand1 = '?' in m
+        if expand1 and name.startswith('_'):
+            raise GrammarError("Inlined rules (_rule) cannot use the ?rule modifier.")
         keep_all_tokens = '!' in m
     else:
         keep_all_tokens = False

tests/test_grammar.py

@@ -281,6 +281,14 @@ def test_list_grammar_imports(self):
             imports = list_grammar_imports('%import common.WS', [])
             assert len(imports) == 1 and imports[0].pkg_name == 'lark'
 
+    def test_inline_with_expand_single(self):
+        grammar = r"""
+        start: _a
+        !?_a: "A"
+        """
+        self.assertRaises(GrammarError, Lark, grammar)
+
+
     def test_line_breaks(self):
         p = Lark(r"""start: "a" \
                        "b"