🎨 Don't double-wrap an integral_constant with ct

Problem:
- Passing an `integral_constant` to `ct` results in a doubly-wrapped
  `integral_constant`.

Solution:
- Partially specialize `ct_helper` to deal with this case.

Author: elbeno

include/stdx/utility.hpp

@@ -260,13 +260,24 @@ constexpr auto is_aligned_with = [](auto v) -> bool {
 };
 
 #if __cplusplus >= 202002L
+template <typename T>
+concept integral_constant_like = requires {
+    []<typename Int, Int I>(std::integral_constant<Int, I>) {}(T{});
+};
 
 namespace detail {
 template <typename T> struct ct_helper {
     // NOLINTNEXTLINE(google-explicit-constructor)
     CONSTEVAL ct_helper(T t) : value(t) {}
     T value;
 };
+
+template <integral_constant_like T> struct ct_helper<T> {
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    CONSTEVAL ct_helper(T) : value(T::value) {}
+    typename T::value_type value;
+};
+
 template <typename T> ct_helper(T) -> ct_helper<T>;
 
 template <auto> CONSTEVAL auto cx_detect0() {}

test/utility.cpp

@@ -260,6 +260,12 @@ TEST_CASE("ct (integral)", "[utility]") {
                        std::integral_constant<unsigned int, 42> const>);
 }
 
+TEST_CASE("ct (integral constant)", "[utility]") {
+    constexpr auto vs = stdx::ct<std::integral_constant<int, 42>{}>();
+    STATIC_REQUIRE(
+        std::is_same_v<decltype(vs), std::integral_constant<int, 42> const>);
+}
+
 TEST_CASE("ct (bool)", "[utility]") {
     constexpr auto v = stdx::ct<true>();
     STATIC_REQUIRE(std::is_same_v<decltype(v), std::bool_constant<true> const>);