From c74437aece4541f501f8a5eda80cb599fc007571 Mon Sep 17 00:00:00 2001
From: Liam Teale <liamteale6@gmail.com>
Date: Tue, 7 Jan 2025 12:18:31 -0800
Subject: [PATCH] add isNan constexpr function

---
 include/units/units.hpp | 7 +++++++
 src/main.cpp            | 2 +-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/units/units.hpp b/include/units/units.hpp
index 0a361eb..044c5d2 100644
--- a/include/units/units.hpp
+++ b/include/units/units.hpp
@@ -413,6 +413,13 @@ NEW_UNIT(Luminosity, candela, 0, 0, 0, 0, 0, 0, 1, 0);
 NEW_UNIT(Moles, mol, 0, 0, 0, 0, 0, 0, 0, 1);
 
 namespace units {
+constexpr bool isNan(double lhs) {
+    // Use the fact that NaN is not equal to itself
+    return lhs != lhs;
+}
+
+template <isQuantity Q> constexpr bool isNan(const Q& lhs) { return isNan(lhs.internal()); }
+
 template <isQuantity Q> constexpr Q abs(const Q& lhs) { return Q(std::abs(lhs.internal())); }
 
 template <isQuantity Q, isQuantity R> constexpr Q max(const Q& lhs, const R& rhs)
diff --git a/src/main.cpp b/src/main.cpp
index 85e4c02..f4e4cee 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -44,7 +44,7 @@ constexpr void sgnTests() {
     static_assert(units::sgn(15_stDeg) == 1);
     static_assert(units::sgn(-15_m) == -1);
     static_assert(units::sgn(0_kg) == 0);
-    static_assert(std::isnan(units::sgn(std::numeric_limits<double>::quiet_NaN())));
+    static_assert(units::isNan(units::sgn(std::numeric_limits<double>::quiet_NaN())));
     static_assert(units::sgn(std::numeric_limits<double>::infinity()) == 1);
     static_assert(units::sgn(-std::numeric_limits<double>::infinity()) == -1);
 }