[GNATS 23855] Spurious diagnostic for no instance of overload function for asio_require_fn

[jhsedg]

Customer bug ID

asio/test/execution/blocking_adaptation and 5 others , gnats/23855.C (preprocessed), 23855a.C (reducded)

Description

​This is the reduced test case:
​
​namespace std {
​template class decay { typedef _Tp type; };
​template <bool, typename = void> struct enable_if;
​template struct enable_if<true, _Tp> { typedef _Tp type; };
​} // namespace std
​using std::decay;
​using std::enable_if;
​namespace asio_require_fn {
​enum overload_type { identity };
​template <typename, typename = void> struct call_traits;
​template
​struct call_traits<
​ void(Property),
​ typename enable_if<decay::type::is_requirable>::type> {
​ static constexpr overload_type overload = identity;
​};
​struct impl {
​ template <typename T, typename Property>
​ typename enable_if<call_traits<void(Property)>::overload == identity>::type
​ operator()(T, Property);
​ void operator()();
​};
​struct static_instance {
​ static impl instance;
​};
​} // namespace asio_require_fn
​namespace blocking_adaptation {
​template <int = 0> struct disallowed_t {
​ static constexpr bool is_requirable = true;
​};
​} // namespace blocking_adaptation
​template <void()> void run_test();
​struct ex_cq_nr {};
​template <typename, typename, typename> void test_require() {
​ asio_require_fn::static_instance::instance(
​ ex_cq_nr(), blocking_adaptation::disallowed_t<>());
​}
​main() { run_test<test_require<ex_cq_nr, int, int>>; }
​
​[ellen@lin1 cwg]$ (nrt)eccp --c++20 -tused /tmp/blocking_adapt.reduced.C
​"/tmp/blocking_adapt.reduced.C", line 35: error: no instance of overloaded
​ function "asio_require_fn::impl::operator()" matches the argument
​ list
​ argument types are: (ex_cq_nr,
​ blocking_adaptation::disallowed_t<0>)
​ object type is: asio_require_fn::impl
​ asio_require_fn::static_instance::instance(
​ ^