Python: Fix tuple parsing for parenthesized element with trailing comma outside (#6811)

The parser incorrectly identified `(` as the tuple's opening paren in
expressions like `("a"),` where the parentheses wrap the element and the
trailing comma outside creates an implicit single-element tuple. This caused
the token index to fall behind the AST position, eventually crashing with
`'NoneType' object has no attribute 'value_source'` when parsing string
literals in type annotations like `Literal["..."]` later in the file.

Author: knutwannheden

rewrite-python/rewrite/src/rewrite/python/_parser_visitor.py

@@ -2691,6 +2691,25 @@ def visit_Tuple(self, node):
                         close_line == second_elt.lineno and close_col < second_elt_char_col
                     ):
                         maybe_parens = False
+                elif close_paren_idx is not None and len(node.elts) == 1:
+                    # For single-element tuples like `("a"),`, check if the '('
+                    # wraps just the element (grouping parens) vs the tuple.
+                    # If the token after ')' is ',' AND there is no ',' inside
+                    # the parens, then '(' is a grouping paren and the trailing
+                    # ',' outside creates the tuple.
+                    # Compare with `("a",),` where ',' is inside the parens
+                    # (making a proper tuple) and the outer ',' belongs to an
+                    # enclosing context.
+                    next_idx = close_paren_idx + 1
+                    while next_idx < len(self._tokens) and self._tokens[next_idx].type in _SKIP_TOKEN_TYPES:
+                        next_idx += 1
+                    if next_idx < len(self._tokens) and self._tokens[next_idx].string == ',':
+                        # Check if there's a comma inside the parens
+                        prev_idx = close_paren_idx - 1
+                        while prev_idx > self._token_idx and self._tokens[prev_idx].type in _SKIP_TOKEN_TYPES:
+                            prev_idx -= 1
+                        if prev_idx > self._token_idx and self._tokens[prev_idx].string != ',':
+                            maybe_parens = False
 
         if maybe_parens:
             self._token_idx += 1  # consume '('

rewrite-python/rewrite/tests/python/all/tree/collection_literal_test.py

@@ -44,6 +44,23 @@ def test_single_element_tuple_with_trailing_comma():
     RecipeSpec().rewrite_run(python("t = (1 , )"))
 
 
+def test_single_element_tuple_with_trailing_comma_outside_parens():
+    # language=python - parenthesized expression with trailing comma outside
+    RecipeSpec().rewrite_run(python('("a"),\n'))
+
+
+def test_single_element_tuple_with_trailing_comma_outside_parens_multiline():
+    # language=python - multi-line parenthesized string with trailing comma outside
+    RecipeSpec().rewrite_run(python(
+        """\
+(
+    "Part 1"
+    " Part 2"
+),
+"""
+    ))
+
+
 def test_tuple_with_first_element_in_parens():
     # language=python
     RecipeSpec().rewrite_run(python("x = (1) // 2, 0"))

rewrite-python/rewrite/tests/python/all/tree/type_hint_test.py

@@ -88,6 +88,16 @@ def test_variable_with_parameterized_type_hint_in_quotes():
     RecipeSpec().rewrite_run(python("""foo: Dict["Foo", str] = None"""))
 
 
+def test_literal_string_type_hint_with_assignment():
+    # language=python - parenthesized string with trailing comma (tuple) before Literal type hint
+    RecipeSpec().rewrite_run(python(
+        """\
+("a"),
+y: Literal["test"] = "value"
+"""
+    ))
+
+
 def test_variable_with_quoted_type_hint():
     # language=python
     RecipeSpec().rewrite_run(python("""foo: 'Foo' = None"""))