Specialize is_nan for ints

Author: WardBrian

stan/math/prim/fun/is_nan.hpp

@@ -8,19 +8,34 @@ namespace stan {
 namespace math {
 
 /**
- * Returns true if the input is NaN and false otherwise.
+ * Tests if the input is Not a Number (NaN)
+ *
+ * Integer specialization, always return false.
+ *
+ * @tparam T Integer type.
+ * @param x Value to test.
+ * @return <code>false</code>.
+ */
+template <typename T, require_integral_t<T>* = nullptr>
+inline bool is_nan(T x) {
+  return false;
+}
+
+/**
+ * Tests if the input is Not a Number (NaN)
  *
  * Delegates to <code>std::isnan</code>.
  *
+ * @tparam T Floating point type.
  * @param x Value to test.
  * @return <code>true</code> if the value is NaN.
  */
-template <typename T, typename = require_arithmetic_t<T>>
+template <typename T, require_floating_point_t<T>* = nullptr>
 inline bool is_nan(T x) {
   return std::isnan(x);
 }
 
-template <typename T, typename = require_eigen_t<T>>
+template <typename T, require_eigen_t<T>* = nullptr>
 inline bool is_nan(const T& x) {
   return x.hasNan();
 }