cast: Qualify symbol usage in PYBIND11_TYPE_CASTER (#3758)

* cast: Qualify symbol usage in PYBIND11_TYPE_CASTER

Permits using macro outside of pybind11::detail

* fixup! review

Author: EricCousineau-TRI

include/pybind11/cast.h

@@ -85,11 +85,15 @@ protected:
                                                                                                   \
 public:                                                                                           \
     static constexpr auto name = py_name;                                                         \
-    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) {                      \
+    template <typename T_,                                                                        \
+              ::pybind11::detail::enable_if_t<                                                    \
+                  std::is_same<type, ::pybind11::detail::remove_cv_t<T_>>::value,                 \
+                  int> = 0>                                                                       \
+    static ::pybind11::handle cast(                                                               \
+        T_ *src, ::pybind11::return_value_policy policy, ::pybind11::handle parent) {             \
         if (!src)                                                                                 \
-            return none().release();                                                              \
-        if (policy == return_value_policy::take_ownership) {                                      \
+            return ::pybind11::none().release();                                                  \
+        if (policy == ::pybind11::return_value_policy::take_ownership) {                          \
             auto h = cast(std::move(*src), policy, parent);                                       \
             delete src;                                                                           \
             return h;                                                                             \
@@ -100,7 +104,7 @@ public:
     operator type &() { return value; }                /* NOLINT(bugprone-macro-parentheses) */   \
     operator type &&() && { return std::move(value); } /* NOLINT(bugprone-macro-parentheses) */   \
     template <typename T_>                                                                        \
-    using cast_op_type = pybind11::detail::movable_cast_op_type<T_>
+    using cast_op_type = ::pybind11::detail::movable_cast_op_type<T_>
 
 template <typename CharT>
 using is_std_char_type = any_of<std::is_same<CharT, char>, /* std::string */

tests/test_custom_type_casters.cpp

@@ -20,6 +20,7 @@ class ArgInspector2 {
     std::string arg = "(default arg inspector 2)";
 };
 class ArgAlwaysConverts {};
+
 namespace pybind11 {
 namespace detail {
 template <>
@@ -105,6 +106,34 @@ struct type_caster<DestructionTester> {
 } // namespace detail
 } // namespace pybind11
 
+// Define type caster outside of `pybind11::detail` and then alias it.
+namespace other_lib {
+struct MyType {};
+// Corrupt `py` shorthand alias for surrounding context.
+namespace py {}
+// Corrupt unqualified relative `pybind11` namespace.
+namespace pybind11 {}
+// Correct alias.
+namespace py_ = ::pybind11;
+// Define caster. This is effectively no-op, we only ensure it compiles and we
+// don't have any symbol collision when using macro mixin.
+struct my_caster {
+    PYBIND11_TYPE_CASTER(MyType, py_::detail::const_name("MyType"));
+    bool load(py_::handle, bool) { return true; }
+
+    static py_::handle cast(const MyType &, py_::return_value_policy, py_::handle) {
+        return py_::bool_(true).release();
+    }
+};
+} // namespace other_lib
+// Effectively "alias" it into correct namespace (via inheritance).
+namespace pybind11 {
+namespace detail {
+template <>
+struct type_caster<other_lib::MyType> : public other_lib::my_caster {};
+} // namespace detail
+} // namespace pybind11
+
 TEST_SUBMODULE(custom_type_casters, m) {
     // test_custom_type_casters
 
@@ -175,4 +204,6 @@ TEST_SUBMODULE(custom_type_casters, m) {
     m.def("destruction_tester_cstats",
           &ConstructorStats::get<DestructionTester>,
           py::return_value_policy::reference);
+
+    m.def("other_lib_type", [](other_lib::MyType x) { return x; });
 }

tests/test_custom_type_casters.py

@@ -115,3 +115,7 @@ def test_custom_caster_destruction():
 
     # Make sure we still only have the original object (from ..._no_destroy()) alive:
     assert cstats.alive() == 1
+
+
+def test_custom_caster_other_lib():
+    assert m.other_lib_type(True)