type_caster_generic: fix compiler error when casting a T that is implicitly convertible from T*

include/pybind11/detail/type_caster_base.h

@@ -563,7 +563,7 @@ struct cast_sources {
     // Use the given pointer with its compile-time type, possibly downcast
     // via polymorphic_type_hook()
     template <typename itype>
-    cast_sources(const itype *ptr); // NOLINT(google-explicit-constructor)
+    explicit cast_sources(const itype *ptr);
 
     // Use the given pointer and type
     // NOLINTNEXTLINE(google-explicit-constructor)
@@ -1621,8 +1621,7 @@ class type_caster_base : public type_caster_generic {
     // that's correct in this context, so you can't use type_caster_base<A>
     // to convert an unrelated B* to Python.
     struct cast_sources : detail::cast_sources {
-        // NOLINTNEXTLINE(google-explicit-constructor)
-        cast_sources(const itype *ptr) : detail::cast_sources(ptr) {}
+        explicit cast_sources(const itype *ptr) : detail::cast_sources(ptr) {}
     };
 
     static handle cast(const itype &src, return_value_policy policy, handle parent) {
@@ -1637,6 +1636,10 @@ class type_caster_base : public type_caster_generic {
         return cast(std::addressof(src), return_value_policy::move, parent);
     }
 
+    static handle cast(const itype *src, return_value_policy policy, handle parent) {
+        return cast(cast_sources{src}, policy, parent);
+    }
+
     static handle cast(const cast_sources &srcs, return_value_policy policy, handle parent) {
         return type_caster_generic::cast(srcs,
                                          policy,
@@ -1645,6 +1648,10 @@ class type_caster_base : public type_caster_generic {
                                          make_move_constructor((const itype *) nullptr));
     }
 
+    static handle cast_holder(const itype *src, const void *holder) {
+        return cast_holder(cast_sources{src}, holder);
+    }
+
     static handle cast_holder(const cast_sources &srcs, const void *holder) {
         auto policy = return_value_policy::take_ownership;
         return type_caster_generic::cast(srcs, policy, {}, nullptr, nullptr, holder);

tests/test_class.cpp

@@ -58,6 +58,13 @@ class ForwardClass;
 class Args : public py::args {};
 } // namespace pr5396_forward_declared_class
 
+struct ConvertibleFromAnything {
+    ConvertibleFromAnything() = default;
+    template <class T>
+    // NOLINTNEXTLINE(bugprone-forwarding-reference-overload,google-explicit-constructor)
+    ConvertibleFromAnything(T &&) {}
+};
+
 } // namespace test_class
 
 static_assert(py::detail::is_same_or_base_of<py::args, py::args>::value, "");
@@ -578,6 +585,11 @@ TEST_SUBMODULE(class_, m) {
     });
 
     test_class::pr4220_tripped_over_this::bind_empty0(m);
+
+    // Regression test for compiler error that showed up in #5866
+    m.def("return_universal_recipient", []() -> test_class::ConvertibleFromAnything {
+        return test_class::ConvertibleFromAnything{};
+    });
 }
 
 template <int N>