From ae963e444fcdf1ad45f662e0c706568f34506528 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Wed, 11 May 2022 07:21:09 -0700
Subject: [PATCH] Implement constexpr isfinite to avoid producing NaN

---
 include/fmt/format.h | 10 +++++++---
 test/format-test.cc  |  2 ++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 9951d1bb..80074f12 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -2343,12 +2343,16 @@ struct has_isfinite<T, enable_if_t<sizeof(std::isfinite(T())) != 0>>
 template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value&&
                                         has_isfinite<T>::value)>
 FMT_CONSTEXPR20 bool isfinite(T value) {
-  if (is_constant_evaluated()) return !isnan(value - value);
+  constexpr T inf = T(std::numeric_limits<double>::infinity());
+  if (is_constant_evaluated())
+    return !isnan(value) && value != inf && value != -inf;
   return std::isfinite(value);
 }
 template <typename T, FMT_ENABLE_IF(!has_isfinite<T>::value)>
-constexpr bool isfinite(T value) {
-  return value - value == 0;  // std::isfinite doesn't support __float128.
+FMT_CONSTEXPR bool isfinite(T value) {
+  T inf = T(std::numeric_limits<double>::infinity());
+  // std::isfinite doesn't support __float128.
+  return !isnan(value) && value != inf && value != -inf;
 }
 
 template <typename T, FMT_ENABLE_IF(is_floating_point<T>::value)>
diff --git a/test/format-test.cc b/test/format-test.cc
index 0743b60c..b4e1e194 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -89,6 +89,8 @@ template <typename Float> void check_isfinite() {
   EXPECT_TRUE(isfinite(Float(fmt::detail::max_value<double>())));
   // Use double because std::numeric_limits is broken for __float128.
   using limits = std::numeric_limits<double>;
+  FMT_CONSTEXPR20 auto result = isfinite(Float(limits::infinity()));
+  EXPECT_FALSE(result);
   EXPECT_FALSE(isfinite(Float(limits::infinity())));
   EXPECT_FALSE(isfinite(Float(-limits::infinity())));
   EXPECT_FALSE(isfinite(Float(limits::quiet_NaN())));