[Syntax] Accept '_' token as a label for tuple expression

Author: rintaro

utils/gyb_syntax_support/AttributeNodes.py

@@ -79,6 +79,7 @@
 
     # attribute-list -> attribute attribute-list?
     Node('AttributeList', kind='SyntaxCollection',
+         omit_when_empty=True,
          element='Syntax', element_name='Attribute',
          element_choices=[
              'Attribute',

utils/gyb_syntax_support/DeclNodes.py

@@ -374,6 +374,7 @@
     #                       | 'weak'
     # mutation-modifier -> 'mutating' | 'nonmutating'
     Node('ModifierList', kind='SyntaxCollection',
+         omit_when_empty=True,
          element='DeclModifier',
          element_name='Modifier'),
 

utils/gyb_syntax_support/ExprNodes.py

@@ -215,8 +215,12 @@
     Node('TupleExprElement', kind='Syntax',
          traits=['WithTrailingComma'],
          children=[
-             Child('Label', kind='IdentifierToken',
-                   is_optional=True),
+             Child('Label', kind='Token',
+                   is_optional=True,
+                   token_choices=[
+                       'IdentifierToken',
+                       'WildcardToken'
+                   ]),
              Child('Colon', kind='ColonToken',
                    is_optional=True),
              Child('Expression', kind='Expr'),