From 1aa8489402c38621daee9a193f19e39814eb9199 Mon Sep 17 00:00:00 2001
From: Alex Dewar <a.dewar@imperial.ac.uk>
Date: Thu, 2 May 2024 14:46:57 +0100
Subject: [PATCH 1/2] Fix: isnan() shouldn't cause FP exceptions

Fixes #3948.
---
 include/fmt/format.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index f4b2c53d13b3..95df0ad00d3b 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -2695,7 +2695,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, const DecimalFP& f,
 }
 
 template <typename T> constexpr auto isnan(T value) -> bool {
-  return !(value >= value);  // std::isnan doesn't support __float128.
+  return value != value;  // std::isnan doesn't support __float128.
 }
 
 template <typename T, typename Enable = void>

From 41e75ee9d5aef98f55e094e19331153fdfd125e1 Mon Sep 17 00:00:00 2001
From: Alex Dewar <a.dewar@imperial.ac.uk>
Date: Thu, 2 May 2024 14:54:23 +0100
Subject: [PATCH 2/2] Add tests to check that isnan doesn't cause FP errors

---
 test/format-test.cc | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/test/format-test.cc b/test/format-test.cc
index 3f5e191ce577..257b6ead7a8f 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -15,6 +15,7 @@
 
 #include <stdint.h>  // uint32_t
 
+#include <cfenv>               // fegetexceptflag and FE_ALL_EXCEPT
 #include <climits>             // INT_MAX
 #include <cmath>               // std::signbit
 #include <condition_variable>  // std::condition_variable
@@ -109,6 +110,14 @@ TEST(float_test, isfinite) {
 #endif
 }
 
+void check_no_fp_exception() {
+  fexcept_t fe;
+  fegetexceptflag(&fe, FE_ALL_EXCEPT);
+
+  // No exception flags should have been set
+  EXPECT_TRUE(fe == 0);
+}
+
 template <typename Float> void check_isnan() {
   using fmt::detail::isnan;
   EXPECT_FALSE(isnan(Float(0.0)));
@@ -121,6 +130,17 @@ template <typename Float> void check_isnan() {
   EXPECT_FALSE(isnan(Float(-limits::infinity())));
   EXPECT_TRUE(isnan(Float(limits::quiet_NaN())));
   EXPECT_TRUE(isnan(Float(-limits::quiet_NaN())));
+
+  // Sanity check: make sure no error has occurred before we start
+  check_no_fp_exception();
+
+  // Check that no exception is raised for the non-NaN case
+  isnan(Float(42.0));
+  check_no_fp_exception();
+
+  // Check that no exception is raised for the NaN case
+  isnan(Float(limits::quiet_NaN()));
+  check_no_fp_exception();
 }
 
 TEST(float_test, isnan) {