libstdc++: Add std::__conditional_t alias template

This change is inspired by the suggestion in
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1715r0.html

The new std::__conditional_t alias template is functionally equivalent
to std::conditional_t but should be more efficient to compile, due to
only ever instantiating two specializations (std::__conditional<true>
and std::__conditional<false>) rather than a new specialization for
every use of std::conditional.

The new alias template is also available in C++11, unlike the C++14
std::conditional_t alias.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__conditional): New class template
	for internal uses of std::conditional.
	(__conditional_t): New alias template to replace conditional_t.
	(__and_, __or_, __result_of_memfun, __result_of_memobj): Use
	__conditional_t instead of conditional::type.
	* include/bits/atomic_base.h (__atomic_impl::_Diff): Likewise.
	* include/bits/hashtable.h (_Hashtable): Likewise.
	* include/bits/hashtable_policy.h (_Node_iterator, _Insert_base)
	(_Local_iterator): Likewise. Replace typedefs with
	using-declarations.
	* include/bits/move.h (move_if_noexcept): Use __conditional_t.
	* include/bits/parse_numbers.h (_Select_int_base): Likewise.
	* include/bits/ptr_traits.h (__make_not_void): Likewise.
	* include/bits/ranges_algobase.h (__copy_or_move_backward)
	(__copy_or_move): Likewise.
	* include/bits/ranges_base.h (borrowed_iterator_t): Likewise.
	* include/bits/ranges_util.h (borrowed_subrange_t): Likewise.
	* include/bits/regex_compiler.h (_BracketMatcher): Use
	__conditional_t. Replace typedefs with using-declarations.
	* include/bits/shared_ptr_base.h (__shared_count): Use
	__conditional_t.
	* include/bits/stl_algobase.h (__copy_move, __copy_move_backward):
	Likewise.
	* include/bits/stl_iterator.h (__detail::__clamp_iter_cat)
	(reverse_iterator::iterator_concept)
	(__make_move_if_noexcept_iterator)
	(iterator_traits<common_iterator<_It, _Sent>>)
	(iterator_traits<counted_iterator<_It>>): Likewise.
	* include/bits/stl_pair.h (_PCC, pair::operator=): Likewise.
	* include/bits/stl_tree.h (_Rb_tree::insert_return_type)
	(_Rb_tree::_M_clone_node): Likewise.
	* include/bits/unique_ptr.h (unique_ptr(unique_ptr<U,E>&&)):
	Likewise.
	* include/bits/uses_allocator.h (__uses_alloc): Likewise.
	(__is_uses_allocator_predicate): Likewise.
	* include/debug/functions.h (__foreign_iterator_aux2): Likewise.
	* include/experimental/any (any::_Manager, __any_caster):
	Likewise.
	* include/experimental/executor (async_completion): Likewise.
	* include/experimental/functional (__boyer_moore_base_t):
	Likewise.
	* include/std/any (any::_Manager): Likewise.
	* include/std/functional (__boyer_moore_base_t): Likewise.
	* include/std/ranges (borrowed_iterator_t)
	(borrowed_subrange_t, __detail::__maybe_present_t)
	(__detail::__maybe_const_t, split_view): Likewise.
	* include/std/tuple (__empty_not_final, tuple::operator=):
	Likewise.
	* include/std/variant (__detail::__variant::__get_t): Likewise.

3 files changed, 9 insertions, 9 deletions

diff --git a/libstdc++-v3/include/experimental/any b/libstdc++-v3/include/experimental/any
index 7d18f267e8b..44f5db0fb87 100644
--- a/libstdc++-v3/include/experimental/any
+++ b/libstdc++-v3/include/experimental/any
@@ -115,9 +115,9 @@ inline namespace fundamentals_v1
       struct _Manager_external; // creates contained object on the heap
 
     template<typename _Tp>
-      using _Manager = conditional_t<_Internal<_Tp>::value,
-				     _Manager_internal<_Tp>,
-				     _Manager_external<_Tp>>;
+      using _Manager = __conditional_t<_Internal<_Tp>::value,
+				       _Manager_internal<_Tp>,
+				       _Manager_external<_Tp>>;
 
     template<typename _Tp, typename _Decayed = decay_t<_Tp>>
       using _Decay = enable_if_t<!is_same<_Decayed, any>::value, _Decayed>;
@@ -430,8 +430,8 @@ inline namespace fundamentals_v1
       // If the type _Tp could never be stored in an any we don't want to
       // instantiate _Manager<_Tp>, so use _Manager<any::_Op> instead, which
       // is explicitly specialized and has a no-op _S_manage function.
-      using _Vp = conditional_t<__and_<__does_not_decay, __is_copyable>::value,
-				_Up, any::_Op>;
+      using _Vp = __conditional_t<__and_<__does_not_decay, __is_copyable>{},
+				  _Up, any::_Op>;
       // First try comparing function addresses, which works without RTTI
       if (__any->_M_manager == &any::_Manager<_Vp>::_S_manage
 #if __cpp_rtti
diff --git a/libstdc++-v3/include/experimental/executor b/libstdc++-v3/include/experimental/executor
index 4322a7f5caf..e7a0c6e6d98 100644
--- a/libstdc++-v3/include/experimental/executor
+++ b/libstdc++-v3/include/experimental/executor
@@ -389,7 +389,7 @@ inline namespace v1
 	= typename __result_type::completion_handler_type;
 
     private:
-      using __handler_type = conditional_t<
+      using __handler_type = __conditional_t<
 	is_same<_CompletionToken, completion_handler_type>::value,
 	completion_handler_type&,
 	completion_handler_type>;
diff --git a/libstdc++-v3/include/experimental/functional b/libstdc++-v3/include/experimental/functional
index 0a2b9381d70..8be2fab1e31 100644
--- a/libstdc++-v3/include/experimental/functional
+++ b/libstdc++-v3/include/experimental/functional
@@ -164,9 +164,9 @@ inline namespace fundamentals_v1
            typename _Val = typename iterator_traits<_RAIter>::value_type,
 	   typename _Diff = typename iterator_traits<_RAIter>::difference_type>
     using __boyer_moore_base_t
-      = std::conditional_t<std::__is_byte_like<_Val, _Pred>::value,
-			   __boyer_moore_array_base<_Diff, 256, _Pred>,
-			   __boyer_moore_map_base<_Val, _Diff, _Hash, _Pred>>;
+      = std::__conditional_t<std::__is_byte_like<_Val, _Pred>::value,
+			     __boyer_moore_array_base<_Diff, 256, _Pred>,
+			     __boyer_moore_map_base<_Val, _Diff, _Hash, _Pred>>;
 
   template<typename _RAIter, typename _Hash
 	     = std::hash<typename std::iterator_traits<_RAIter>::value_type>,