[PATCH] Minor simplification to std::_Bind_result helpers

[Jonathan Wakely]

We don't need to define new class templates for the SFINAE helpers in
_Bind_result, we can just use alias templates. This also moves where
the helpers are used to the return types, instead of as a defaulted
argument.

        * include/std/functional (_Bind_result::__enable_if_void): Use alias
        template instead of class template.
        (_Bind_result::__disable_if_void): Likewise.
        (_Bind_result::__call): Adjust uses of __enable_if_void and
        __disable_if_void.

Tested powerpc64le-linux, committed to trunk.


commit 1330ba1b3b4ccddc64e532756aa2f571f27ae2ad
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Oct 10 17:00:49 2016 +0100

    Minor simplification to std::_Bind_result helpers
    
        * include/std/functional (_Bind_result::__enable_if_void): Use alias
        template instead of class template.
        (_Bind_result::__disable_if_void): Likewise.
        (_Bind_result::__call): Adjust uses of __enable_if_void and
        __disable_if_void.

diff --git a/libstdc++-v3/include/std/functional 
b/libstdc++-v3/include/std/functional
index 1c7523e..2587392 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -1000,15 +1000,17 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // sfinae types
       template<typename _Res>
-       struct __enable_if_void : enable_if<is_void<_Res>::value, int> { };
+       using __enable_if_void
+         = typename enable_if<is_void<_Res>{}>::type;
+
       template<typename _Res>
-       struct __disable_if_void : enable_if<!is_void<_Res>::value, int> { };
+       using __disable_if_void
+         = typename enable_if<!is_void<_Res>{}, _Result>::type;
 
       // Call unqualified
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       _Result
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __disable_if_void<_Res>::type = 0)
+       __disable_if_void<_Res>
+       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>)
        {
          return _M_f(_Mu<_Bound_args>()
                      (std::get<_Indexes>(_M_bound_args), __args)...);
@@ -1016,9 +1018,8 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call unqualified, return void
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       void
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __enable_if_void<_Res>::type = 0)
+       __enable_if_void<_Res>
+       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>)
        {
          _M_f(_Mu<_Bound_args>()
               (std::get<_Indexes>(_M_bound_args), __args)...);
@@ -1026,9 +1027,8 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call as const
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       _Result
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __disable_if_void<_Res>::type = 0) const
+       __disable_if_void<_Res>
+       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>) const
        {
          return _M_f(_Mu<_Bound_args>()
                      (std::get<_Indexes>(_M_bound_args), __args)...);
@@ -1036,9 +1036,8 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call as const, return void
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       void
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __enable_if_void<_Res>::type = 0) const
+       __enable_if_void<_Res>
+       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>) const
        {
          _M_f(_Mu<_Bound_args>()
               (std::get<_Indexes>(_M_bound_args),  __args)...);
@@ -1046,9 +1045,8 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call as volatile
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       _Result
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __disable_if_void<_Res>::type = 0) volatile
+       __disable_if_void<_Res>
+       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>) volatile
        {
          return _M_f(_Mu<_Bound_args>()
                      (__volget<_Indexes>(_M_bound_args), __args)...);
@@ -1056,9 +1054,8 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call as volatile, return void
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       void
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __enable_if_void<_Res>::type = 0) volatile
+       __enable_if_void<_Res>
+       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>) volatile
        {
          _M_f(_Mu<_Bound_args>()
               (__volget<_Indexes>(_M_bound_args), __args)...);
@@ -1066,9 +1063,9 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call as const volatile
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       _Result
-       __call(tuple<_Args...>&& __args, _Index_tuple<_Indexes...>,
-           typename __disable_if_void<_Res>::type = 0) const volatile
+       __disable_if_void<_Res>
+       __call(tuple<_Args...>&& __args,
+              _Index_tuple<_Indexes...>) const volatile
        {
          return _M_f(_Mu<_Bound_args>()
                      (__volget<_Indexes>(_M_bound_args), __args)...);
@@ -1076,10 +1073,9 @@ _GLIBCXX_MEM_FN_TRAITS(&&, false_type, true_type)
 
       // Call as const volatile, return void
       template<typename _Res, typename... _Args, std::size_t... _Indexes>
-       void
+       __enable_if_void<_Res>
        __call(tuple<_Args...>&& __args,
-              _Index_tuple<_Indexes...>,
-           typename __enable_if_void<_Res>::type = 0) const volatile
+              _Index_tuple<_Indexes...>) const volatile
        {
          _M_f(_Mu<_Bound_args>()
               (__volget<_Indexes>(_M_bound_args), __args)...);