🐛 Fix CX_WRAP for empty types

Problem:
- When `CX_WRAP` wraps empty types, it doesn't turn them into
  `type_identity<T>`.

Solution:
- Move the emptiness check after the type check.

Author: elbeno

include/stdx/utility.hpp

@@ -297,8 +297,7 @@ constexpr auto cx_detect1(auto) { return 0; }
         STDX_PRAGMA(diagnostic push)                                           \
         STDX_PRAGMA(diagnostic ignored "-Wold-style-cast")                     \
         if constexpr (::stdx::is_cx_value_v<                                   \
-                          std::invoke_result_t<decltype(f)>> or                \
-                      std::is_empty_v<std::invoke_result_t<decltype(f)>>) {    \
+                          std::invoke_result_t<decltype(f)>>) {                \
             return f();                                                        \
         } else if constexpr (CX_DETECT(X)) {                                   \
             if constexpr (decltype(::stdx::cxv_detail::is_type<                \
@@ -310,6 +309,9 @@ constexpr auto cx_detect1(auto) { return 0; }
                             decltype(::stdx::cxv_detail::type_of<              \
                                      ::stdx::cxv_detail::from_any(X)>())>{};   \
                     }};                                                        \
+            } else if constexpr (std::is_empty_v<                              \
+                                     std::invoke_result_t<decltype(f)>>) {     \
+                return f();                                                    \
             } else {                                                           \
                 return ::stdx::overload{::stdx::cxv_detail::cx_base{}, f};     \
             }                                                                  \

test/utility.cpp

@@ -364,6 +364,13 @@ TEST_CASE("CX_WRAP type argument", "[utility]") {
         std::is_same_v<decltype(CX_WRAP(int)()), stdx::type_identity<int>>);
 }
 
+TEST_CASE("CX_WRAP empty type argument", "[utility]") {
+    using X = std::integral_constant<int, 17>;
+    STATIC_REQUIRE(stdx::is_cx_value_v<decltype(CX_WRAP(X))>);
+    STATIC_REQUIRE(
+        std::is_same_v<decltype(CX_WRAP(X)()), stdx::type_identity<X>>);
+}
+
 TEST_CASE("CX_WRAP integral_constant arg", "[utility]") {
     auto x = std::integral_constant<int, 17>{};
     STATIC_REQUIRE(std::is_same_v<decltype(CX_WRAP(x)), decltype(x)>);