feat(types) Adds special Case for empty C++ tuple type annotation (#5214)

* add special case and unit test

* add newline

Author: InvincibleRMC

include/pybind11/cast.h

@@ -740,6 +740,13 @@ class type_caster<std::pair<T1, T2>> : public tuple_caster<std::pair, T1, T2> {}
 template <typename... Ts>
 class type_caster<std::tuple<Ts...>> : public tuple_caster<std::tuple, Ts...> {};
 
+template <>
+class type_caster<std::tuple<>> : public tuple_caster<std::tuple> {
+public:
+    // PEP 484 specifies this syntax for an empty tuple
+    static constexpr auto name = const_name("tuple[()]");
+};
+
 /// Helper class which abstracts away certain actions. Users can provide specializations for
 /// custom holders, but it's only necessary if the type has a non-standard interface.
 template <typename T>

tests/test_builtin_casters.py

@@ -368,6 +368,8 @@ def test_tuple(doc):
     """
     )
 
+    assert doc(m.empty_tuple) == """empty_tuple() -> tuple[()]"""
+
     assert m.rvalue_pair() == ("rvalue", "rvalue")
     assert m.lvalue_pair() == ("lvalue", "lvalue")
     assert m.rvalue_tuple() == ("rvalue", "rvalue", "rvalue")