Fix regression: container pointers not castable

PR #936 broke the ability to return a pointer to a stl container (and,
likewise, to a tuple) because the added deduced type matched a
non-const pointer argument: the pointer-accepting `cast` in
PYBIND11_TYPE_CASTER had a `const type *`, which is a worse match for a
non-const pointer than the universal reference template #936 added.

This changes the provided TYPE_CASTER cast(ptr) to take the pointer by
template arg (so that it will accept either const or non-const pointer).
It has two other effects: it slightly reduces .so size (because many
type casters never actually need the pointer cast at all), and it allows
type casters to provide their untemplated pointer `cast()` that will
take precedence over the templated version provided in the macro.

include/pybind11/cast.h

@@ -892,14 +892,15 @@ public:
         type value; \
     public: \
         static PYBIND11_DESCR name() { return type_descr(py_name); } \
-        static handle cast(const type *src, return_value_policy policy, handle parent) { \
+        template <typename T_, enable_if_t<std::is_same<type, remove_cv_t<T_>>::value, int> = 0> \
+        static handle cast(T_ *src, return_value_policy policy, handle parent) { \
             if (!src) return none().release(); \
             return cast(*src, policy, parent); \
         } \
         operator type*() { return &value; } \
         operator type&() { return value; } \
         operator type&&() && { return std::move(value); } \
-        template <typename _T> using cast_op_type = pybind11::detail::movable_cast_op_type<_T>
+        template <typename T_> using cast_op_type = pybind11::detail::movable_cast_op_type<T_>
 
 
 template <typename CharT> using is_std_char_type = any_of<

tests/test_stl.cpp

@@ -38,6 +38,9 @@ TEST_SUBMODULE(stl, m) {
     // test_vector
     m.def("cast_vector", []() { return std::vector<int>{1}; });
     m.def("load_vector", [](const std::vector<int> &v) { return v.at(0) == 1 && v.at(1) == 2; });
+    // Unnumbered regression (caused by #936): pointers to stl containers aren't castable
+    static std::vector<RValueCaster> lvv{2};
+    m.def("cast_ptr_vector", []() { return &lvv; });
 
     // test_array
     m.def("cast_array", []() { return std::array<int, 2> {{1 , 2}}; });
@@ -78,7 +81,6 @@ TEST_SUBMODULE(stl, m) {
         v.back()[1].back().emplace("a", RValueCaster{});
         return v;
     });
-    static std::vector<RValueCaster> lvv{2};
     static std::array<RValueCaster, 2> lva;
     static std::unordered_map<std::string, RValueCaster> lvm{{"a", RValueCaster{}}, {"b", RValueCaster{}}};
     static std::unordered_map<std::string, std::vector<std::list<std::array<RValueCaster, 2>>>> lvn;

tests/test_stl.py

@@ -15,6 +15,9 @@ def test_vector(doc):
     assert doc(m.cast_vector) == "cast_vector() -> List[int]"
     assert doc(m.load_vector) == "load_vector(arg0: List[int]) -> bool"
 
+    # Test regression caused by 936: pointers to stl containers weren't castable
+    assert m.cast_ptr_vector() == ["lvalue", "lvalue"]
+
 
 def test_array(doc):
     """std::array <-> list"""