add isNan constexpr function

Author: SizzinSeal

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)

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);
 }