[libc++][NFC] Simplify the implementation of __promote

Author: philnik777

libcxx/include/__type_traits/promote.h

@@ -11,18 +11,18 @@
 
 #include <__config>
 #include <__type_traits/integral_constant.h>
-#include <__type_traits/is_same.h>
-#include <__utility/declval.h>
+#include <__type_traits/is_arithmetic.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _Tp>
-struct __numeric_type {
-  static void __test(...);
+template <class... _Args>
+class __promote {
+  static_assert((is_arithmetic<_Args>::value && ...));
+
   static float __test(float);
   static double __test(char);
   static double __test(int);
@@ -38,57 +38,10 @@ struct __numeric_type {
   static double __test(double);
   static long double __test(long double);
 
-  typedef decltype(__test(std::declval<_Tp>())) type;
-  static const bool value = _IsNotSame<type, void>::value;
-};
-
-template <>
-struct __numeric_type<void> {
-  static const bool value = true;
-};
-
-template <class _A1,
-          class _A2 = void,
-          class _A3 = void,
-          bool      = __numeric_type<_A1>::value && __numeric_type<_A2>::value && __numeric_type<_A3>::value>
-class __promote_imp {
-public:
-  static const bool value = false;
-};
-
-template <class _A1, class _A2, class _A3>
-class __promote_imp<_A1, _A2, _A3, true> {
-private:
-  typedef typename __promote_imp<_A1>::type __type1;
-  typedef typename __promote_imp<_A2>::type __type2;
-  typedef typename __promote_imp<_A3>::type __type3;
-
 public:
-  typedef decltype(__type1() + __type2() + __type3()) type;
-  static const bool value = true;
+  using type = decltype((__test(_Args()) + ...));
 };
 
-template <class _A1, class _A2>
-class __promote_imp<_A1, _A2, void, true> {
-private:
-  typedef typename __promote_imp<_A1>::type __type1;
-  typedef typename __promote_imp<_A2>::type __type2;
-
-public:
-  typedef decltype(__type1() + __type2()) type;
-  static const bool value = true;
-};
-
-template <class _A1>
-class __promote_imp<_A1, void, void, true> {
-public:
-  typedef typename __numeric_type<_A1>::type type;
-  static const bool value = true;
-};
-
-template <class _A1, class _A2 = void, class _A3 = void>
-class __promote : public __promote_imp<_A1, _A2, _A3> {};
-
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___TYPE_TRAITS_PROMOTE_H