From 300b952454c7a57a61ae58775518491747682dcd Mon Sep 17 00:00:00 2001
From: Ben Deane <ben@elbeno.com>
Date: Wed, 25 Feb 2026 16:00:24 -0700
Subject: [PATCH] :art: Don't double-wrap an `integral_constant` with `ct`

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

Solution:
- Add a deduction guide so that `ct_helper` deals with this case.
---
 include/stdx/utility.hpp | 6 ++++--
 test/utility.cpp         | 6 ++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/stdx/utility.hpp b/include/stdx/utility.hpp
index 18ebd01..20db122 100644
--- a/include/stdx/utility.hpp
+++ b/include/stdx/utility.hpp
@@ -260,14 +260,16 @@ constexpr auto is_aligned_with = [](auto v) -> bool {
 };
 
 #if __cplusplus >= 202002L
-
 namespace detail {
 template <typename T> struct ct_helper {
     // NOLINTNEXTLINE(google-explicit-constructor)
-    CONSTEVAL ct_helper(T t) : value(t) {}
+    CONSTEVAL ct_helper(auto t) : value(t) {}
     T value;
 };
+
 template <typename T> ct_helper(T) -> ct_helper<T>;
+template <typename T, T I>
+ct_helper(std::integral_constant<T, I>) -> ct_helper<T>;
 
 template <auto> CONSTEVAL auto cx_detect0() {}
 CONSTEVAL auto cx_detect1(auto) { return 0; }
diff --git a/test/utility.cpp b/test/utility.cpp
index a7eceec..1fa2c46 100644
--- a/test/utility.cpp
+++ b/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>);