Improve diagnostics

Author: mingxwa

include/proxy/v4/proxy.h

@@ -465,10 +465,9 @@ template <class T, class... Args>
 using accessor_t = typename a11y_traits<void, T, Args...>::type;
 
 template <class C, class F, class... Os>
-struct conv_traits_impl : inapplicable_traits {};
-template <class C, class F, class... Os>
-  requires(overload_traits<substituted_overload_t<Os, F>>::applicable && ...)
-struct conv_traits_impl<C, F, Os...> : applicable_traits {
+struct conv_traits_impl {
+  static_assert((overload_traits<substituted_overload_t<Os, F>>::applicable &&
+                 ...));
   using meta =
       composite_meta<invocation_meta<F, C::is_direct, typename C::dispatch_type,
                                      substituted_overload_t<Os, F>>...>;
@@ -640,6 +639,12 @@ struct ptr_traits<P> : applicable_traits {};
 template <class F>
 struct ptr_traits<proxy<F>> : inapplicable_traits {};
 
+template <class P>
+consteval bool diagnose_proxiable_pointer() {
+  constexpr bool verdict = ptr_traits<P>::applicable;
+  static_assert(verdict, "only pointer or pointer-like types are proxiable");
+  return verdict;
+}
 template <class P, class F, std::size_t ActualSize, std::size_t MaxSize>
 consteval bool diagnose_proxiable_size_too_large() {
   constexpr bool verdict = ActualSize <= MaxSize;
@@ -727,10 +732,7 @@ template <class F>
 struct basic_facade_traits<F> : applicable_traits {};
 
 template <class F, class... Cs>
-struct facade_conv_traits_impl : inapplicable_traits {};
-template <class F, class... Cs>
-  requires(conv_traits<Cs, F>::applicable && ...)
-struct facade_conv_traits_impl<F, Cs...> : applicable_traits {
+struct facade_conv_traits_impl {
   using conv_meta =
       composite_t<composite_meta<>, typename conv_traits<Cs, F>::meta...>;
   using conv_indirect_accessor =
@@ -772,11 +774,7 @@ struct facade_refl_traits_impl {
       (refl_traits<Rs>::template applicable_ptr<P> && ...);
 };
 template <class F>
-struct facade_traits : inapplicable_traits {};
-template <class F>
-  requires(instantiated_t<facade_conv_traits_impl, typename F::convention_types,
-                          F>::applicable)
-struct facade_traits<F>
+struct facade_traits
     : instantiated_t<facade_conv_traits_impl, typename F::convention_types, F>,
       instantiated_t<facade_refl_traits_impl, typename F::reflection_types, F> {
   using meta = composite_t<
@@ -797,7 +795,7 @@ struct facade_traits<F>
 
   template <class P>
   static consteval void diagnose_proxiable() {
-    bool verdict = true;
+    bool verdict = diagnose_proxiable_pointer<P>();
     verdict &=
         diagnose_proxiable_size_too_large<P, F, sizeof(P), F::max_size>();
     verdict &=
@@ -819,7 +817,8 @@ struct facade_traits<F>
 
   template <class P>
   static constexpr bool applicable_ptr =
-      sizeof(P) <= F::max_size && alignof(P) <= F::max_align &&
+      ptr_traits<P>::applicable && sizeof(P) <= F::max_size &&
+      alignof(P) <= F::max_align &&
       copyability_traits<P, F::copyability>::applicable &&
       relocatability_traits<P, F::relocatability>::applicable &&
       destructibility_traits<P, F::destructibility>::applicable &&
@@ -913,9 +912,8 @@ add_qualifier_t<proxy<F>, Q>
 } // namespace details
 
 template <class P, class F>
-concept proxiable = facade<F> && details::facade_traits<F>::applicable &&
-                    details::ptr_traits<P>::applicable &&
-                    details::facade_traits<F>::template applicable_ptr<P>;
+concept proxiable =
+    facade<F> && details::facade_traits<F>::template applicable_ptr<P>;
 
 template <facade F>
 class proxy_indirect_accessor
@@ -930,7 +928,6 @@ template <facade F>
 class proxy : public details::facade_traits<F>::direct_accessor,
               public details::inplace_ptr<proxy_indirect_accessor<F>> {
   friend struct details::proxy_helper;
-  static_assert(details::facade_traits<F>::applicable);
 
 public:
   using facade_type = F;