fix: render empty-tuple subscript annotation as tuple[()] not tuple[]

packages/griffelib/src/griffe/_internal/expressions.py

@@ -1003,6 +1003,14 @@ class ExprTuple(Expr):
     """Whether the tuple is implicit (e.g. without parentheses in a subscript's slice)."""
 
     def iterate(self, *, flat: bool = True) -> Iterator[str | Expr]:
+        if self.implicit and not self.elements:
+            # An empty tuple is always written as `()` and cannot be implicit.
+            # This arises in annotations like `tuple[()]`, where the AST represents
+            # the subscript slice as an empty Tuple node, but the parentheses must
+            # be preserved to produce valid Python (`tuple[]` is a SyntaxError).
+            yield "("
+            yield ")"
+            return
         if not self.implicit:
             yield "("
         yield from _join(self.elements, ", ", flat=flat)

packages/griffelib/tests/test_expressions.py

@@ -311,3 +311,22 @@ def test_render_dict_with_unpacking() -> None:
         assert str(module["a"].value) == "{**base, 'x': 1}"
         assert str(module["b"].value) == "{**d1, **d2}"
         assert str(module["c"].value) == "{None: 1, 'y': 2}"
+
+
+def test_empty_tuple_annotation_str() -> None:
+    """Check that empty-tuple annotations round-trip correctly.
+
+    ``tuple[()]`` is a valid annotation for a zero-element tuple.
+    Its string representation must remain ``tuple[()]``, not the
+    invalid ``tuple[]``.
+    """
+    with temporary_visited_module(
+        """
+        from typing import Tuple
+
+        def f1() -> tuple[()]: ...
+        def f2() -> Tuple[()]: ...
+        """,
+    ) as module:
+        assert str(module["f1"].returns) == "tuple[()]"
+        assert str(module["f2"].returns) == "Tuple[()]"