[libc++][math.h] Remove unnecessary uses of __promote

[philnik]

Summary

Removes __promote when it's just the identity.

[EricWF]

What happens here with __fp16 or other extended floating point types.
Users have been known to specialize is_floating_point to make these things work.

[ldionne]

In D136868#3892502, @EricWF wrote:

What happens here with __fp16 or other extended floating point types.
Users have been known to specialize is_floating_point to make these things work.

Users are not allowed to do that because __fp16 is not a user-defined type, so they can't specialize a type trait in namespace std AFAICT. And if someone did do that, TBH it would be broken in way more ways than just this.

IMO the path forward here for custom floating point types is to do http://wg21.link/p1467 and consider the problem in its entirety.

[ldionne]

Just wondering -- did you notice any issues caused by this change specifically?

[EricWF]

Yes. There are older version of this library that override
is_floating_point for the half type it provides.
Though it looks like newer versions have a different approach.
https://sourceforge.net/projects/half/

libcxx/include/math.h

 
 #undef signbit
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 signbit(_A1 __x) _NOEXCEPT
 {
-    return __libcpp_signbit((typename std::__promote<_A1>::type)__x);
+    return __libcpp_signbit(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<
     std::is_integral<_A1>::value && std::is_signed<_A1>::value, bool>::type
 signbit(_A1 __x) _NOEXCEPT
 { return __x < 0; }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<
     std::is_integral<_A1>::value && !std::is_signed<_A1>::value, bool>::type
 signbit(_A1) _NOEXCEPT
 { return false; }
 
 #elif defined(_LIBCPP_MSVCRT)
 
 template <typename _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 signbit(_A1 __x) _NOEXCEPT
 {
-  return ::signbit(static_cast<typename std::__promote<_A1>::type>(__x));
+  return ::signbit(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<
     std::is_integral<_A1>::value && std::is_signed<_A1>::value, bool>::type
 signbit(_A1 __x) _NOEXCEPT
 { return __x < 0; }

 
 #undef fpclassify
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_floating_point<_A1>::value, int>::type
 fpclassify(_A1 __x) _NOEXCEPT
 {
-    return __libcpp_fpclassify((typename std::__promote<_A1>::type)__x);
+    return __libcpp_fpclassify(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_integral<_A1>::value, int>::type
 fpclassify(_A1 __x) _NOEXCEPT
 { return __x == 0 ? FP_ZERO : FP_NORMAL; }
 
 #elif defined(_LIBCPP_MSVCRT)
 
 template <typename _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 fpclassify(_A1 __x) _NOEXCEPT
 {
-  return ::fpclassify(static_cast<typename std::__promote<_A1>::type>(__x));
+  return ::fpclassify(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_integral<_A1>::value, int>::type
 fpclassify(_A1 __x) _NOEXCEPT
 { return __x == 0 ? FP_ZERO : FP_NORMAL; }
 

 
 #undef isnan
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 isnan(_A1 __x) _NOEXCEPT
 {
-    return __libcpp_isnan((typename std::__promote<_A1>::type)__x);
+    return __libcpp_isnan(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_integral<_A1>::value, bool>::type
 isnan(_A1) _NOEXCEPT
 { return false; }
 

 
 #undef isnormal
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_floating_point<_A1>::value, bool>::type
 isnormal(_A1 __x) _NOEXCEPT
 {
-    return __libcpp_isnormal((typename std::__promote<_A1>::type)__x);
+    return __libcpp_isnormal(__x);
 }
 
 template <class _A1>
 inline _LIBCPP_HIDE_FROM_ABI
 typename std::enable_if<std::is_integral<_A1>::value, bool>::type
 isnormal(_A1 __x) _NOEXCEPT
 { return __x != 0; }