fixup! Replace global map holders_seen with local holder_type in registered type_info

Having replaced the global map holders_seen, we can only check return values against already
registered types. Hence, we need to replace the check at initialization time with a check at call
time (when casting the return value).

Author: rhaschke

include/pybind11/cast.h

@@ -1538,6 +1538,7 @@ struct copyable_holder_caster : public type_caster_base<type> {
     explicit operator holder_type&() { return holder; }
 
     static handle cast(const holder_type &src, return_value_policy, handle) {
+        check_for_holder_mismatch_impl<holder_type>();
         const auto *ptr = holder_helper<holder_type>::get(src);
         return type_caster_base<type>::cast_holder(ptr, &src);
     }

include/pybind11/pybind11.h

@@ -157,7 +157,7 @@ class cpp_function : public function {
         static_assert(expected_num_args<Extra...>(sizeof...(Args), cast_in::has_args, cast_in::has_kwargs),
                       "The number of argument annotations does not match the number of function arguments");
 
-        // Fail if we've previously seen a different holder around the held type
+        // Fail if the type was previously registered with a different holder type
         detail::check_for_holder_mismatch<Return>();
 
         /* Dispatch code which converts function arguments and performs the actual function call */
@@ -1225,9 +1225,6 @@ class class_ : public detail::generic_type {
                 none_of<std::is_same<multiple_inheritance, Extra>...>::value), // no multiple_inheritance attr
             "Error: multiple inheritance bases must be specified via class_ template options");
 
-        // Fail if we've previously seen a different holder around the type
-        detail::check_for_holder_mismatch<holder_type>();
-
         type_record record;
         record.scope = scope;
         record.name = name;

tests/test_smart_ptr.cpp

@@ -378,8 +378,7 @@ TEST_SUBMODULE(smart_ptr, m) {
         m.def("bad1", []() { return std::unique_ptr<HeldByShared>(new HeldByShared()); });
     });
     m.def("return_shared", []() { return std::make_shared<HeldByUnique>(); });
-    m.def("register_mismatch_class", [](py::module m) {
-        // Fails: `return_shared' already returned this via shared_ptr holder
+    m.def("register_HeldByUnique", [](py::module m) {
         py::class_<HeldByUnique>(m, "HeldByUnique");
     });
     // Fails: MyObject5 was declared with huge_unique_ptr as holder instead of shared_ptr

tests/test_smart_ptr.py

@@ -316,8 +316,11 @@ def test_holder_mismatch():
         m.register_mismatch_return(m)
     assert "Mismatched holders detected" in str(excinfo)
 
+    with pytest.raises(TypeError) as excinfo:
+        m.return_shared()  # calling a function returning an unregistered type
+    m.register_HeldByUnique(m)  # register the type
     with pytest.raises(RuntimeError) as excinfo:
-        m.register_mismatch_class(m)
+        m.return_shared()  # calling a function returning a mismatching holder type
     assert "Mismatched holders detected" in str(excinfo)
 
     with pytest.raises(RuntimeError) as excinfo: