Merge pull request #235 from AntelopeIO/remove_callable_v_workaround

[4.0 -> main] merge up #233: remove `is_callable_v<>` workaround

libraries/meta_refl/include/bluegrass/meta/function_traits.hpp

@@ -76,6 +76,9 @@ namespace bluegrass { namespace meta {
       inline constexpr U&& make_dependent(U&& u) { return static_cast<U&&>(u); }
    }
 
+   template <typename F>
+   constexpr bool is_callable(F&& fn) { return BLUEGRASS_HAS_MEMBER(fn, operator()); }
+
    namespace detail {
       template <bool Decay, typename R, typename... Args>
       constexpr auto get_types(R(Args...)) -> std::tuple<R, freestanding,
@@ -100,7 +103,7 @@ namespace bluegrass { namespace meta {
                                                                std::tuple<std::conditional_t<Decay, std::decay_t<Args>, Args>...>>;
       template <bool Decay, typename F>
       constexpr auto get_types(F&& fn) {
-         if constexpr (std::is_invocable<F>::value)
+         if constexpr (is_callable(fn))
             return get_types<Decay>(&F::operator());
          else
             return get_types<Decay>(fn);
@@ -142,7 +145,7 @@ namespace bluegrass { namespace meta {
       constexpr auto parameters_from_impl(R(Cls::*)(Args...)const &&) ->  pack_from_t<N, Args...>;
       template <std::size_t N, typename F>
       constexpr auto parameters_from_impl(F&& fn) {
-         if constexpr (std::is_invocable<F>::value)
+         if constexpr (is_callable(fn))
             return parameters_from_impl<N>(&F::operator());
          else
             return parameters_from_impl<N>(fn);