https://github.com/fmtlib/fmt/issues/4417

From 9db5e4df22d47ea6741913b5ca618687133494fe Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Sat, 26 Apr 2025 08:17:05 -0700
Subject: [PATCH] Don't specialize std::is_floating_point

---
 include/fmt/format.h     | 11 +++++------
 test/format-impl-test.cc |  4 ++--
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 2596e3a8..b2fdd3d6 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -721,11 +721,10 @@ struct float128 {};
 
 template <typename T> using is_float128 = std::is_same<T, float128>;
 
-template <typename T>
-using is_floating_point =
-    bool_constant<std::is_floating_point<T>::value || is_float128<T>::value>;
+template <typename T> struct is_floating_point : std::is_floating_point<T> {};
+template <> struct is_floating_point<float128> : std::true_type {};
 
-template <typename T, bool = std::is_floating_point<T>::value>
+template <typename T, bool = is_floating_point<T>::value>
 struct is_fast_float : bool_constant<std::numeric_limits<T>::is_iec559 &&
                                      sizeof(T) <= sizeof(double)> {};
 template <typename T> struct is_fast_float<T, false> : std::false_type {};
@@ -2472,8 +2471,8 @@ template <typename T>
 struct has_isfinite<T, enable_if_t<sizeof(std::isfinite(T())) != 0>>
     : std::true_type {};
 
-template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value&&
-                                        has_isfinite<T>::value)>
+template <typename T,
+          FMT_ENABLE_IF(is_floating_point<T>::value&& has_isfinite<T>::value)>
 FMT_CONSTEXPR20 auto isfinite(T value) -> bool {
   constexpr T inf = T(std::numeric_limits<double>::infinity());
   if (is_constant_evaluated())
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index 4ebf2a6e..2be15d00 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -307,7 +307,6 @@ struct slow_float {
 auto format_as(slow_float f) -> float { return f; }
 
 namespace std {
-template <> struct is_floating_point<double_double> : std::true_type {};
 template <> struct numeric_limits<double_double> {
   // is_iec559 is true for double-double in libstdc++.
   static constexpr bool is_iec559 = true;
@@ -315,12 +314,13 @@ template <> struct numeric_limits<double_double> {
   static constexpr int digits10 = 33;
 };
 
-template <> struct is_floating_point<slow_float> : std::true_type {};
 template <> struct numeric_limits<slow_float> : numeric_limits<float> {};
 }  // namespace std
 
 FMT_BEGIN_NAMESPACE
 namespace detail {
+template <> struct is_floating_point<double_double> : std::true_type {};
+template <> struct is_floating_point<slow_float> : std::true_type {};
 template <> struct is_fast_float<slow_float> : std::false_type {};
 namespace dragonbox {
 template <> struct float_info<slow_float> {
-- 
2.49.1