[libc++][NFC] Rename _EnableIf to __enable_if_t for consistency

[ldionne]

Summary

In other places in the code, we use lowercase spelling for things that
are not available in prior standards.

[• Quuxplusone]

Seems reasonable if this is the direction we end up going. I think you should hold off until we figure out whether this needs to be _LIBCPP_ENABLEIF. (I think that depends only on whether we care about good diagnostics on Clang 13.)

[• Quuxplusone]

Per my latest comment/update on D109411: I buy this now. We're switching away from the SCARY metabase technique (which irreparably interferes with the good diagnostics) at the same time that we're switching away from the _EnableIf name. Simultaneously, on the Clang side, we're special-casing the new name, __enable_if_t, so that it will start giving the good diagnostics.

(I think it would still be nice to decide whether we are going to use __enable_if_t throughout for consistency, or permit _LIBCPP_STD_VER > 11 codepaths to use enable_if_t if they want. I predict the latter will win out anyway, though.)

LGTM, ship it! Assuming you agree with D109411 at this point too.

[ldionne]

In D109435#2989549, @Quuxplusone wrote:

Per my latest comment/update on D109411: I buy this now. We're switching away from the SCARY metabase technique (which irreparably interferes with the good diagnostics) at the same time that we're switching away from the _EnableIf name. Simultaneously, on the Clang side, we're special-casing the new name, __enable_if_t, so that it will start giving the good diagnostics.

(I think it would still be nice to decide whether we are going to use __enable_if_t throughout for consistency, or permit _LIBCPP_STD_VER > 11 codepaths to use enable_if_t if they want. I predict the latter will win out anyway, though.)

My opinion is that we should use enable_if_t where we can, and __enable_if_t when we can't because the std version doesn't permit us to do so. It's what we do for all other utilities, e.g. std::__to_address & friends, so that approach seems consistent to me.

LGTM, ship it! Assuming you agree with D109411 at this point too.

Yes, I agree with D109411 (just approved it).

libcxx/include/__functional/bind.h

 typename __invoke_of<_Ti&, _Uj...>::type
 __mu_expand(_Ti& __ti, tuple<_Uj...>& __uj, __tuple_indices<_Indx...>)
 {
     return __ti(_VSTD::forward<_Uj>(_VSTD::get<_Indx>(__uj))...);
 }
 
 template <class _Ti, class ..._Uj>
 inline _LIBCPP_INLINE_VISIBILITY
-typename _EnableIf
+typename __enable_if_t
 <
     is_bind_expression<_Ti>::value,
     __invoke_of<_Ti&, _Uj...>
 >::type
 __mu(_Ti& __ti, tuple<_Uj...>& __uj)
 {
     typedef typename __make_tuple_indices<sizeof...(_Uj)>::type __indices;
     return _VSTD::__mu_expand(__ti, __uj, __indices());

libcxx/include/__functional/reference_wrapper.h

 
 public:
     // construct/copy/destroy
 #ifdef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     reference_wrapper(type& __f) _NOEXCEPT
         : __f_(_VSTD::addressof(__f)) {}
 #else
-    template <class _Up, class = _EnableIf<!__is_same_uncvref<_Up, reference_wrapper>::value, decltype(__fun(declval<_Up>())) >>
+    template <class _Up, class = __enable_if_t<!__is_same_uncvref<_Up, reference_wrapper>::value, decltype(__fun(declval<_Up>())) >>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     reference_wrapper(_Up&& __u) _NOEXCEPT_(noexcept(__fun(declval<_Up>()))) {
         type& __f = static_cast<_Up&&>(__u);
         __f_ = _VSTD::addressof(__f);
     }
 #endif
 
     // access

libcxx/include/__iterator/advance.h

 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX14
 void __advance(_RandIter& __i, typename iterator_traits<_RandIter>::difference_type __n, random_access_iterator_tag) {
   __i += __n;
 }
 
 template <
     class _InputIter, class _Distance,
     class _IntegralDistance = decltype(_VSTD::__convert_to_integral(declval<_Distance>())),
-    class = _EnableIf<is_integral<_IntegralDistance>::value> >
+    class = __enable_if_t<is_integral<_IntegralDistance>::value> >
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX14
 void advance(_InputIter& __i, _Distance __orig_n) {
   typedef typename iterator_traits<_InputIter>::difference_type _Difference;
   _Difference __n = static_cast<_Difference>(_VSTD::__convert_to_integral(__orig_n));
   _LIBCPP_ASSERT(__n >= 0 || __is_cpp17_bidirectional_iterator<_InputIter>::value,
                  "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
   _VSTD::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
 }

libcxx/include/__iterator/iterator_traits.h

struct __iter_concept_concept_test {
   using _Apply = typename _ITER_TRAITS<_Iter>::iterator_concept;
 };
 struct __iter_concept_category_test {
   template <class _Iter>
   using _Apply = typename _ITER_TRAITS<_Iter>::iterator_category;
 };
 struct __iter_concept_random_fallback {
   template <class _Iter>
-  using _Apply = _EnableIf<
+  using _Apply = __enable_if_t<
                           __is_primary_template<iterator_traits<_Iter> >::value,
                           random_access_iterator_tag
                         >;
 };
 
 template <class _Iter, class _Tester> struct __test_iter_concept
     : _IsValidExpansion<_Tester::template _Apply, _Iter>,
       _Tester

libcxx/include/__iterator/move_iterator.h

 #endif
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     move_iterator() : __i() {}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     explicit move_iterator(_Iter __x) : __i(__x) {}
 
-    template <class _Up, class = _EnableIf<
+    template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value && is_convertible<_Up const&, _Iter>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     move_iterator(const move_iterator<_Up>& __u) : __i(__u.base()) {}
 
-    template <class _Up, class = _EnableIf<
+    template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value &&
         is_convertible<_Up const&, _Iter>::value &&
         is_assignable<_Iter&, _Up const&>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     move_iterator& operator=(const move_iterator<_Up>& __u) {
         __i = __u.base();
         return *this;

libcxx/include/__iterator/reverse_iterator.h

 
 #ifndef _LIBCPP_ABI_NO_ITERATOR_BASES
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator() : __t(), current() {}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     explicit reverse_iterator(_Iter __x) : __t(__x), current(__x) {}
 
-    template <class _Up, class = _EnableIf<
+    template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value && is_convertible<_Up const&, _Iter>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator(const reverse_iterator<_Up>& __u)
         : __t(__u.base()), current(__u.base())
     { }
 
-    template <class _Up, class = _EnableIf<
+    template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value &&
         is_convertible<_Up const&, _Iter>::value &&
         is_assignable<_Up const&, _Iter>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator& operator=(const reverse_iterator<_Up>& __u) {
         __t = current = __u.base();
         return *this;
     }
 #else
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator() : current() {}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     explicit reverse_iterator(_Iter __x) : current(__x) {}
 
-    template <class _Up, class = _EnableIf<
+    template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value && is_convertible<_Up const&, _Iter>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator(const reverse_iterator<_Up>& __u)
         : current(__u.base())
     { }
 
-    template <class _Up, class = _EnableIf<
+    template <class _Up, class = __enable_if_t<
         !is_same<_Up, _Iter>::value &&
         is_convertible<_Up const&, _Iter>::value &&
         is_assignable<_Up const&, _Iter>::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reverse_iterator& operator=(const reverse_iterator<_Up>& __u) {
         current = __u.base();
         return *this;

libcxx/include/__memory/allocator_traits.h

 #endif // _LIBCPP_CXX03_LANG
 
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer allocate(allocator_type& __a, size_type __n) {
         return __a.allocate(__n);
     }
 
     template <class _Ap = _Alloc, class =
-        _EnableIf<__has_allocate_hint<_Ap, size_type, const_void_pointer>::value> >
+        __enable_if_t<__has_allocate_hint<_Ap, size_type, const_void_pointer>::value> >
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer allocate(allocator_type& __a, size_type __n, const_void_pointer __hint) {
         _LIBCPP_SUPPRESS_DEPRECATED_PUSH
         return __a.allocate(__n, __hint);
         _LIBCPP_SUPPRESS_DEPRECATED_POP
     }
     template <class _Ap = _Alloc, class = void, class =
-        _EnableIf<!__has_allocate_hint<_Ap, size_type, const_void_pointer>::value> >
+        __enable_if_t<!__has_allocate_hint<_Ap, size_type, const_void_pointer>::value> >
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static pointer allocate(allocator_type& __a, size_type __n, const_void_pointer) {
         return __a.allocate(__n);
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static void deallocate(allocator_type& __a, pointer __p, size_type __n) _NOEXCEPT {
         __a.deallocate(__p, __n);
     }
 
     template <class _Tp, class... _Args, class =
-        _EnableIf<__has_construct<allocator_type, _Tp*, _Args...>::value> >
+        __enable_if_t<__has_construct<allocator_type, _Tp*, _Args...>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static void construct(allocator_type& __a, _Tp* __p, _Args&&... __args) {
         _LIBCPP_SUPPRESS_DEPRECATED_PUSH
         __a.construct(__p, _VSTD::forward<_Args>(__args)...);
         _LIBCPP_SUPPRESS_DEPRECATED_POP
     }
     template <class _Tp, class... _Args, class = void, class =
-        _EnableIf<!__has_construct<allocator_type, _Tp*, _Args...>::value> >
+        __enable_if_t<!__has_construct<allocator_type, _Tp*, _Args...>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static void construct(allocator_type&, _Tp* __p, _Args&&... __args) {
 #if _LIBCPP_STD_VER > 17
         _VSTD::construct_at(__p, _VSTD::forward<_Args>(__args)...);
 #else
         ::new ((void*)__p) _Tp(_VSTD::forward<_Args>(__args)...);
 #endif
     }
 
     template <class _Tp, class =
-        _EnableIf<__has_destroy<allocator_type, _Tp*>::value> >
+        __enable_if_t<__has_destroy<allocator_type, _Tp*>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static void destroy(allocator_type& __a, _Tp* __p) {
         _LIBCPP_SUPPRESS_DEPRECATED_PUSH
         __a.destroy(__p);
         _LIBCPP_SUPPRESS_DEPRECATED_POP
     }
     template <class _Tp, class = void, class =
-        _EnableIf<!__has_destroy<allocator_type, _Tp*>::value> >
+        __enable_if_t<!__has_destroy<allocator_type, _Tp*>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static void destroy(allocator_type&, _Tp* __p) {
 #if _LIBCPP_STD_VER > 17
         _VSTD::destroy_at(__p);
 #else
         __p->~_Tp();
 #endif
     }
 
     template <class _Ap = _Alloc, class =
-        _EnableIf<__has_max_size<const _Ap>::value> >
+        __enable_if_t<__has_max_size<const _Ap>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static size_type max_size(const allocator_type& __a) _NOEXCEPT {
         _LIBCPP_SUPPRESS_DEPRECATED_PUSH
         return __a.max_size();
         _LIBCPP_SUPPRESS_DEPRECATED_POP
     }
     template <class _Ap = _Alloc, class = void, class =
-        _EnableIf<!__has_max_size<const _Ap>::value> >
+        __enable_if_t<!__has_max_size<const _Ap>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static size_type max_size(const allocator_type&) _NOEXCEPT {
         return numeric_limits<size_type>::max() / sizeof(value_type);
     }
 
     template <class _Ap = _Alloc, class =
-        _EnableIf<__has_select_on_container_copy_construction<const _Ap>::value> >
+        __enable_if_t<__has_select_on_container_copy_construction<const _Ap>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static allocator_type select_on_container_copy_construction(const allocator_type& __a) {
         return __a.select_on_container_copy_construction();
     }
     template <class _Ap = _Alloc, class = void, class =
-        _EnableIf<!__has_select_on_container_copy_construction<const _Ap>::value> >
+        __enable_if_t<!__has_select_on_container_copy_construction<const _Ap>::value> >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     static allocator_type select_on_container_copy_construction(const allocator_type& __a) {
         return __a;
     }
 };
 
 template <class _Traits, class _Tp>
 struct __rebind_alloc_helper {

 
 // __is_cpp17_move_insertable
 template <class _Alloc, class = void>
 struct __is_cpp17_move_insertable
     : is_move_constructible<typename _Alloc::value_type>
 { };
 
 template <class _Alloc>
-struct __is_cpp17_move_insertable<_Alloc, _EnableIf<
+struct __is_cpp17_move_insertable<_Alloc, __enable_if_t<
     !__is_default_allocator<_Alloc>::value &&
     __has_construct<_Alloc, typename _Alloc::value_type*, typename _Alloc::value_type&&>::value
 > > : true_type { };
 
 // __is_cpp17_copy_insertable
 template <class _Alloc, class = void>
 struct __is_cpp17_copy_insertable
     : integral_constant<bool,
         is_copy_constructible<typename _Alloc::value_type>::value &&
         __is_cpp17_move_insertable<_Alloc>::value
     >
 { };
 
 template <class _Alloc>
-struct __is_cpp17_copy_insertable<_Alloc, _EnableIf<
+struct __is_cpp17_copy_insertable<_Alloc, __enable_if_t<
     !__is_default_allocator<_Alloc>::value &&
     __has_construct<_Alloc, typename _Alloc::value_type*, const typename _Alloc::value_type&>::value
 > >
     : __is_cpp17_move_insertable<_Alloc>
 { };
 
 #undef _LIBCPP_ALLOCATOR_TRAITS_HAS_XXX
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___MEMORY_ALLOCATOR_TRAITS_H

libcxx/include/__memory/pointer_traits.h

 template <class _Tp>
 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Tp* __to_address(_Tp* __p) _NOEXCEPT {
     static_assert(!is_function<_Tp>::value, "_Tp is a function type");
     return __p;
 }
 
 // enable_if is needed here to avoid instantiating checks for fancy pointers on raw pointers
-template <class _Pointer, class = _EnableIf<
+template <class _Pointer, class = __enable_if_t<
     !is_pointer<_Pointer>::value && !is_array<_Pointer>::value && !is_function<_Pointer>::value
 > >
 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 typename decay<decltype(__to_address_helper<_Pointer>::__call(declval<const _Pointer&>()))>::type
 __to_address(const _Pointer& __p) _NOEXCEPT {
     return __to_address_helper<_Pointer>::__call(__p);
 }
 

libcxx/include/__memory/shared_ptr.h

private:
 
     struct __nat {int __for_bool_;};
 public:
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR shared_ptr() _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR shared_ptr(nullptr_t) _NOEXCEPT;
 
-    template<class _Yp, class = _EnableIf<
+    template<class _Yp, class = __enable_if_t<
         _And<
             __compatible_with<_Yp, _Tp>
             // In C++03 we get errors when trying to do SFINAE with the
             // delete operator, so we always pretend that it's deletable.
             // The same happens on GCC.
 #if !defined(_LIBCPP_CXX03_LANG) && !defined(_LIBCPP_COMPILER_GCC)
             , _If<is_array<_Tp>::value, __is_array_deletable<_Yp*>, __is_deletable<_Yp*> >
 #endif

 shared_ptr<_Tp>::reset(_Yp* __p, _Dp __d, _Alloc __a)
 {
     shared_ptr(__p, __d, __a).swap(*this);
 }
 
 //
 // std::allocate_shared and std::make_shared
 //
-template<class _Tp, class _Alloc, class ..._Args, class = _EnableIf<!is_array<_Tp>::value> >
+template<class _Tp, class _Alloc, class ..._Args, class = __enable_if_t<!is_array<_Tp>::value> >
 _LIBCPP_HIDE_FROM_ABI
 shared_ptr<_Tp> allocate_shared(const _Alloc& __a, _Args&& ...__args)
 {
     using _ControlBlock = __shared_ptr_emplace<_Tp, _Alloc>;
     using _ControlBlockAllocator = typename __allocator_traits_rebind<_Alloc, _ControlBlock>::type;
     __allocation_guard<_ControlBlockAllocator> __guard(__a, 1);
     ::new ((void*)_VSTD::addressof(*__guard.__get())) _ControlBlock(__a, _VSTD::forward<_Args>(__args)...);
     auto __control_block = __guard.__release_ptr();
     return shared_ptr<_Tp>::__create_with_control_block((*__control_block).__get_elem(), _VSTD::addressof(*__control_block));
 }
 
-template<class _Tp, class ..._Args, class = _EnableIf<!is_array<_Tp>::value> >
+template<class _Tp, class ..._Args, class = __enable_if_t<!is_array<_Tp>::value> >
 _LIBCPP_HIDE_FROM_ABI
 shared_ptr<_Tp> make_shared(_Args&& ...__args)
 {
     return _VSTD::allocate_shared<_Tp>(allocator<_Tp>(), _VSTD::forward<_Args>(__args)...);
 }
 
 template<class _Tp, class _Up>
 inline _LIBCPP_INLINE_VISIBILITY

libcxx/include/bitset

 public:
     typedef typename base::reference       reference;
     typedef typename base::const_reference const_reference;
 
     // 23.3.5.1 constructors:
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR bitset() _NOEXCEPT {}
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
         bitset(unsigned long long __v) _NOEXCEPT : base(__v) {}
-    template<class _CharT, class = _EnableIf<_IsCharLikeType<_CharT>::value> >
+    template<class _CharT, class = __enable_if_t<_IsCharLikeType<_CharT>::value> >
         explicit bitset(const _CharT* __str,
                         typename basic_string<_CharT>::size_type __n = basic_string<_CharT>::npos,
                         _CharT __zero = _CharT('0'), _CharT __one = _CharT('1'));
     template<class _CharT, class _Traits, class _Allocator>
         explicit bitset(const basic_string<_CharT,_Traits,_Allocator>& __str,
                         typename basic_string<_CharT,_Traits,_Allocator>::size_type __pos = 0,
                         typename basic_string<_CharT,_Traits,_Allocator>::size_type __n =
                                 (basic_string<_CharT,_Traits,_Allocator>::npos),

libcxx/include/math.h

 
 #if !(defined(_AIX) || defined(__sun__))
 inline _LIBCPP_INLINE_VISIBILITY float       atan2(float __lcpp_y, float __lcpp_x) _NOEXCEPT             {return ::atan2f(__lcpp_y, __lcpp_x);}
 inline _LIBCPP_INLINE_VISIBILITY long double atan2(long double __lcpp_y, long double __lcpp_x) _NOEXCEPT {return ::atan2l(__lcpp_y, __lcpp_x);}
 #endif
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 atan2(_A1 __lcpp_y, _A2 __lcpp_x) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;

 
 #if !(defined(_AIX) || defined(__sun__))
 inline _LIBCPP_INLINE_VISIBILITY float       fmod(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::fmodf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double fmod(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::fmodl(__lcpp_x, __lcpp_y);}
 #endif
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 fmod(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;

 
 #if !(defined(_AIX) || defined(__sun__))
 inline _LIBCPP_INLINE_VISIBILITY float       pow(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::powf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double pow(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::powl(__lcpp_x, __lcpp_y);}
 #endif
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 pow(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;

 #endif
 }
 
 template <class _A1, class _A2>
 #if __has_builtin(__builtin_copysign)
 _LIBCPP_CONSTEXPR
 #endif
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 __libcpp_copysign(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT {
     typedef typename std::__promote<_A1, _A2>::type __result_type;
     static_assert((!(std::_IsSame<_A1, __result_type>::value &&

 }
 
 inline _LIBCPP_INLINE_VISIBILITY long double copysign(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {
   return ::__libcpp_copysign(__lcpp_x, __lcpp_y);
 }
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
     copysign(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT {
   return ::__libcpp_copysign(__lcpp_x, __lcpp_y);
 }

 
 // fdim
 
 inline _LIBCPP_INLINE_VISIBILITY float       fdim(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::fdimf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double fdim(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::fdiml(__lcpp_x, __lcpp_y);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 fdim(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;
#if __has_builtin(__builtin_fmal)
     return __builtin_fmal(__lcpp_x, __lcpp_y, __lcpp_z);
 #else
     return ::fmal(__lcpp_x, __lcpp_y, __lcpp_z);
 #endif
 }
 
 template <class _A1, class _A2, class _A3>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value &&
     std::is_arithmetic<_A3>::value,
     std::__promote<_A1, _A2, _A3>
 >::type
 fma(_A1 __lcpp_x, _A2 __lcpp_y, _A3 __lcpp_z) _NOEXCEPT
 {

 
 // fmax
 
 inline _LIBCPP_INLINE_VISIBILITY float       fmax(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::fmaxf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double fmax(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::fmaxl(__lcpp_x, __lcpp_y);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 fmax(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;
     static_assert((!(std::_IsSame<_A1, __result_type>::value &&
                      std::_IsSame<_A2, __result_type>::value)), "");
     return ::fmax((__result_type)__lcpp_x, (__result_type)__lcpp_y);
 }
 
 // fmin
 
 inline _LIBCPP_INLINE_VISIBILITY float       fmin(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::fminf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double fmin(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::fminl(__lcpp_x, __lcpp_y);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 fmin(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;
     static_assert((!(std::_IsSame<_A1, __result_type>::value &&
                      std::_IsSame<_A2, __result_type>::value)), "");
     return ::fmin((__result_type)__lcpp_x, (__result_type)__lcpp_y);
 }
 
 // hypot
 
 inline _LIBCPP_INLINE_VISIBILITY float       hypot(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::hypotf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double hypot(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::hypotl(__lcpp_x, __lcpp_y);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 hypot(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;

 
 // nextafter
 
 inline _LIBCPP_INLINE_VISIBILITY float       nextafter(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::nextafterf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double nextafter(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::nextafterl(__lcpp_x, __lcpp_y);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 nextafter(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;

 
 // remainder
 
 inline _LIBCPP_INLINE_VISIBILITY float       remainder(float __lcpp_x, float __lcpp_y) _NOEXCEPT             {return ::remainderf(__lcpp_x, __lcpp_y);}
 inline _LIBCPP_INLINE_VISIBILITY long double remainder(long double __lcpp_x, long double __lcpp_y) _NOEXCEPT {return ::remainderl(__lcpp_x, __lcpp_y);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 remainder(_A1 __lcpp_x, _A2 __lcpp_y) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;
     static_assert((!(std::_IsSame<_A1, __result_type>::value &&
                      std::_IsSame<_A2, __result_type>::value)), "");
     return ::remainder((__result_type)__lcpp_x, (__result_type)__lcpp_y);
 }
 
 // remquo
 
 inline _LIBCPP_INLINE_VISIBILITY float       remquo(float __lcpp_x, float __lcpp_y, int* __lcpp_z) _NOEXCEPT             {return ::remquof(__lcpp_x, __lcpp_y, __lcpp_z);}
 inline _LIBCPP_INLINE_VISIBILITY long double remquo(long double __lcpp_x, long double __lcpp_y, int* __lcpp_z) _NOEXCEPT {return ::remquol(__lcpp_x, __lcpp_y, __lcpp_z);}
 
 template <class _A1, class _A2>
 inline _LIBCPP_INLINE_VISIBILITY
-typename std::_EnableIf
+typename std::__enable_if_t
 <
     std::is_arithmetic<_A1>::value &&
     std::is_arithmetic<_A2>::value,
     std::__promote<_A1, _A2>
 >::type
 remquo(_A1 __lcpp_x, _A2 __lcpp_y, int* __lcpp_z) _NOEXCEPT
 {
     typedef typename std::__promote<_A1, _A2>::type __result_type;

libcxx/include/queue

#endif // _LIBCPP_CXX03_LANG
     explicit queue(const container_type& __c)  : c(__c) {}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     explicit queue(container_type&& __c) : c(_VSTD::move(__c)) {}
 #endif // _LIBCPP_CXX03_LANG
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         explicit queue(const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         queue(const queue& __q, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__q.c, __a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         queue(const container_type& __c, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__c, __a) {}
 #ifndef _LIBCPP_CXX03_LANG
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         queue(container_type&& __c, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(_VSTD::move(__c), __a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         queue(queue&& __q, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(_VSTD::move(__q.c), __a) {}
 
 #endif // _LIBCPP_CXX03_LANG
 
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
     bool      empty() const {return c.empty();}
     _LIBCPP_INLINE_VISIBILITY
     size_type size() const  {return c.size();}

 bool
 operator<=(const queue<_Tp, _Container>& __x,const queue<_Tp, _Container>& __y)
 {
     return !(__y < __x);
 }
 
 template <class _Tp, class _Container>
 inline _LIBCPP_INLINE_VISIBILITY
-_EnableIf<__is_swappable<_Container>::value, void>
+__enable_if_t<__is_swappable<_Container>::value, void>
 swap(queue<_Tp, _Container>& __x, queue<_Tp, _Container>& __y)
     _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
 {
     __x.swap(__y);
 }
 
 template <class _Tp, class _Container, class _Alloc>
 struct _LIBCPP_TEMPLATE_VIS uses_allocator<queue<_Tp, _Container>, _Alloc>
#endif // _LIBCPP_CXX03_LANG
     explicit priority_queue(const value_compare& __comp)
         : c(), comp(__comp) {}
     _LIBCPP_INLINE_VISIBILITY
     priority_queue(const value_compare& __comp, const container_type& __c);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     priority_queue(const value_compare& __comp, container_type&& __c);
 #endif
-    template <class _InputIter, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l,
                        const value_compare& __comp = value_compare());
-    template <class _InputIter, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l,
                        const value_compare& __comp, const container_type& __c);
 #ifndef _LIBCPP_CXX03_LANG
-    template <class _InputIter, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l,
                        const value_compare& __comp, container_type&& __c);
 #endif // _LIBCPP_CXX03_LANG
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         explicit priority_queue(const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(const value_compare& __comp, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(const value_compare& __comp, const container_type& __c,
                        const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(const priority_queue& __q, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
 #ifndef _LIBCPP_CXX03_LANG
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(const value_compare& __comp, container_type&& __c,
                        const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(priority_queue&& __q, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
 #endif // _LIBCPP_CXX03_LANG
 
-    template <class _InputIter, class _Alloc, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class _Alloc, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
 
-    template <class _InputIter, class _Alloc, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class _Alloc, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l,
                        const value_compare& __comp, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
 
-    template <class _InputIter, class _Alloc, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class _Alloc, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l,
                        const value_compare& __comp, const container_type& __c, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
 
 #ifndef _LIBCPP_CXX03_LANG
-    template <class _InputIter, class _Alloc, class = _EnableIf<__is_cpp17_input_iterator<_InputIter>::value> >
+    template <class _InputIter, class _Alloc, class = __enable_if_t<__is_cpp17_input_iterator<_InputIter>::value> >
         _LIBCPP_INLINE_VISIBILITY
         priority_queue(_InputIter __f, _InputIter __l,
                        const value_compare& __comp, container_type&& __c, const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0);
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0);
 #endif  // _LIBCPP_CXX03_LANG
 
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
     bool            empty() const {return c.empty();}
     _LIBCPP_INLINE_VISIBILITY
     size_type       size() const  {return c.size();}
     _LIBCPP_INLINE_VISIBILITY
     const_reference top() const   {return c.front();}

 }
 
 #endif // _LIBCPP_CXX03_LANG
 
 template <class _Tp, class _Container, class _Compare>
 template <class _Alloc>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__a)
 {
 }
 
 template <class _Tp, class _Container, class _Compare>
 template <class _Alloc>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(const value_compare& __comp,
                                                           const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__a),
       comp(__comp)
 {
 }
 
 template <class _Tp, class _Container, class _Compare>
 template <class _Alloc>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(const value_compare& __comp,
                                                           const container_type& __c,
                                                           const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__c, __a),
       comp(__comp)
 {
     _VSTD::make_heap(c.begin(), c.end(), comp);
 }
 
 template <class _Tp, class _Container, class _Compare>
 template <class _Alloc>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(const priority_queue& __q,
                                                           const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__q.c, __a),
       comp(__q.comp)
 {
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 
 template <class _Tp, class _Container, class _Compare>
 template <class _Alloc>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(const value_compare& __comp,
                                                           container_type&& __c,
                                                           const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(_VSTD::move(__c), __a),
       comp(__comp)
 {
     _VSTD::make_heap(c.begin(), c.end(), comp);
 }
 
 template <class _Tp, class _Container, class _Compare>
 template <class _Alloc>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(priority_queue&& __q,
                                                           const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(_VSTD::move(__q.c), __a),
       comp(_VSTD::move(__q.comp))
 {
 }
 
 #endif  // _LIBCPP_CXX03_LANG
 
 template <class _Tp, class _Container, class _Compare>
 template <class _InputIter, class _Alloc, class>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(
         _InputIter __f, _InputIter __l, const _Alloc& __a,
-        _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+        __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__f, __l, __a),
       comp()
 {
     _VSTD::make_heap(c.begin(), c.end(), comp);
 }
 
 template <class _Tp, class _Container, class _Compare>
 template <class _InputIter, class _Alloc, class>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(
         _InputIter __f, _InputIter __l,
         const value_compare& __comp, const _Alloc& __a,
-        _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+        __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__f, __l, __a),
       comp(__comp)
 {
     _VSTD::make_heap(c.begin(), c.end(), comp);
 }
 
 template <class _Tp, class _Container, class _Compare>
 template <class _InputIter, class _Alloc, class>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(
         _InputIter __f, _InputIter __l,
         const value_compare& __comp, const container_type& __c, const _Alloc& __a,
-        _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+        __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(__c, __a),
       comp(__comp)
 {
     c.insert(c.end(), __f, __l);
     _VSTD::make_heap(c.begin(), c.end(), comp);
 }
 
 #ifndef _LIBCPP_CXX03_LANG
 template <class _Tp, class _Container, class _Compare>
 template <class _InputIter, class _Alloc, class>
 inline
 priority_queue<_Tp, _Container, _Compare>::priority_queue(
         _InputIter __f, _InputIter __l, const value_compare& __comp,
         container_type&& __c, const _Alloc& __a,
-        _EnableIf<uses_allocator<container_type, _Alloc>::value>*)
+        __enable_if_t<uses_allocator<container_type, _Alloc>::value>*)
     : c(_VSTD::move(__c), __a),
       comp(__comp)
 {
     c.insert(c.end(), __f, __l);
     _VSTD::make_heap(c.begin(), c.end(), comp);
 }
 #endif  // _LIBCPP_CXX03_LANG
 

 {
     using _VSTD::swap;
     swap(c, __q.c);
     swap(comp, __q.comp);
 }
 
 template <class _Tp, class _Container, class _Compare>
 inline _LIBCPP_INLINE_VISIBILITY
-_EnableIf<
+__enable_if_t<
     __is_swappable<_Container>::value && __is_swappable<_Compare>::value,
     void
 >
 swap(priority_queue<_Tp, _Container, _Compare>& __x,
      priority_queue<_Tp, _Container, _Compare>& __y)
     _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
 {
     __x.swap(__y);

libcxx/include/stack

 #endif // _LIBCPP_CXX03_LANG
 
     _LIBCPP_INLINE_VISIBILITY
     explicit stack(const container_type& __c) : c(__c) {}
 
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         explicit stack(const _Alloc& __a,
-                       _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+                       __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(const container_type& __c, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__c, __a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(const stack& __s, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(__s.c, __a) {}
 #ifndef _LIBCPP_CXX03_LANG
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(container_type&& __c, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(_VSTD::move(__c), __a) {}
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY
         stack(stack&& __s, const _Alloc& __a,
-              _EnableIf<uses_allocator<container_type, _Alloc>::value>* = 0)
+              __enable_if_t<uses_allocator<container_type, _Alloc>::value>* = 0)
             : c(_VSTD::move(__s.c), __a) {}
 #endif // _LIBCPP_CXX03_LANG
 
     _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY
     bool empty()     const      {return c.empty();}
     _LIBCPP_INLINE_VISIBILITY
     size_type size() const      {return c.size();}
     _LIBCPP_INLINE_VISIBILITY

 bool
 operator<=(const stack<_Tp, _Container>& __x, const stack<_Tp, _Container>& __y)
 {
     return !(__y < __x);
 }
 
 template <class _Tp, class _Container>
 inline _LIBCPP_INLINE_VISIBILITY
-_EnableIf<__is_swappable<_Container>::value, void>
+__enable_if_t<__is_swappable<_Container>::value, void>
 swap(stack<_Tp, _Container>& __x, stack<_Tp, _Container>& __y)
     _NOEXCEPT_(_NOEXCEPT_(__x.swap(__y)))
 {
     __x.swap(__y);
 }
 
 template <class _Tp, class _Container, class _Alloc>
 struct _LIBCPP_TEMPLATE_VIS uses_allocator<stack<_Tp, _Container>, _Alloc>
     : public uses_allocator<_Container, _Alloc>
 {
 };
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STACK

libcxx/include/string

 #else
         _NOEXCEPT;
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string(basic_string&& __str, const allocator_type& __a);
 #endif // _LIBCPP_CXX03_LANG
 
-    template <class = _EnableIf<__is_allocator<_Allocator>::value, nullptr_t> >
+    template <class = __enable_if_t<__is_allocator<_Allocator>::value, nullptr_t> >
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s) : __r_(__default_init_tag(), __default_init_tag()) {
       _LIBCPP_ASSERT(__s != nullptr, "basic_string(const char*) detected nullptr");
       __init(__s, traits_type::length(__s));
 #   if _LIBCPP_DEBUG_LEVEL == 2
       __get_db()->__insert_c(this);
 #   endif
     }
 
-    template <class = _EnableIf<__is_allocator<_Allocator>::value, nullptr_t> >
+    template <class = __enable_if_t<__is_allocator<_Allocator>::value, nullptr_t> >
         _LIBCPP_INLINE_VISIBILITY
         basic_string(const _CharT* __s, const _Allocator& __a);
 
 #if _LIBCPP_STD_VER > 20
     basic_string(nullptr_t) = delete;
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s, size_type __n);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const _CharT* __s, size_type __n, const _Allocator& __a);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(size_type __n, _CharT __c);
 
-    template <class = _EnableIf<__is_allocator<_Allocator>::value, nullptr_t> >
+    template <class = __enable_if_t<__is_allocator<_Allocator>::value, nullptr_t> >
         _LIBCPP_INLINE_VISIBILITY
         basic_string(size_type __n, _CharT __c, const _Allocator& __a);
 
     basic_string(const basic_string& __str, size_type __pos, size_type __n,
                  const _Allocator& __a = _Allocator());
     _LIBCPP_INLINE_VISIBILITY
     basic_string(const basic_string& __str, size_type __pos,
                  const _Allocator& __a = _Allocator());
 
-    template<class _Tp, class = _EnableIf<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
+    template<class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
         _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
         basic_string(const _Tp& __t, size_type __pos, size_type __n,
                      const allocator_type& __a = allocator_type());
 
-    template<class _Tp, class = _EnableIf<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value &&
+    template<class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value &&
                                           !__is_same_uncvref<_Tp, basic_string>::value> >
         _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
         explicit basic_string(const _Tp& __t);
 
-    template<class _Tp, class = _EnableIf<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
+    template<class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
         _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
         explicit basic_string(const _Tp& __t, const allocator_type& __a);
 
-    template<class _InputIterator, class = _EnableIf<__is_cpp17_input_iterator<_InputIterator>::value> >
+    template<class _InputIterator, class = __enable_if_t<__is_cpp17_input_iterator<_InputIterator>::value> >
         _LIBCPP_INLINE_VISIBILITY
         basic_string(_InputIterator __first, _InputIterator __last);
-    template<class _InputIterator, class = _EnableIf<__is_cpp17_input_iterator<_InputIterator>::value> >
+    template<class _InputIterator, class = __enable_if_t<__is_cpp17_input_iterator<_InputIterator>::value> >
         _LIBCPP_INLINE_VISIBILITY
         basic_string(_InputIterator __first, _InputIterator __last, const allocator_type& __a);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string(initializer_list<_CharT> __il);
     _LIBCPP_INLINE_VISIBILITY
     basic_string(initializer_list<_CharT> __il, const _Allocator& __a);
 #endif // _LIBCPP_CXX03_LANG
 
     inline ~basic_string();
 
     _LIBCPP_INLINE_VISIBILITY
     operator __self_view() const _NOEXCEPT { return __self_view(data(), size()); }
 
     basic_string& operator=(const basic_string& __str);
 
-    template <class _Tp, class = _EnableIf<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
+    template <class _Tp, class = __enable_if_t<__can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value> >
     basic_string& operator=(const _Tp& __t)
         {__self_view __sv = __t; return assign(__sv);}
 
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string& operator=(basic_string&& __str)
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value));
      _LIBCPP_INLINE_VISIBILITY
#endif // _LIBCPP_DEBUG_LEVEL == 2
 
     const_reference at(size_type __n) const;
     reference       at(size_type __n);
 
     _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(const basic_string& __str) {return append(__str);}
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string >::value,
             basic_string&
         >
                                             operator+=(const _Tp& __t)            {__self_view __sv = __t; return append(__sv);}
     _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(const value_type* __s)     {return append(__s);}
     _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(value_type __c)            {push_back(__c); return *this;}
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY basic_string& operator+=(initializer_list<value_type> __il) {return append(__il);}
 #endif // _LIBCPP_CXX03_LANG
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string& append(const basic_string& __str);
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf<
+    __enable_if_t<
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   append(const _Tp& __t) { __self_view __sv = __t; return append(__sv.data(), __sv.size()); }
     basic_string& append(const basic_string& __str, size_type __pos, size_type __n=npos);
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   append(const _Tp& __t, size_type __pos, size_type __n=npos);
     basic_string& append(const value_type* __s, size_type __n);
     basic_string& append(const value_type* __s);
     basic_string& append(size_type __n, value_type __c);
 
     _LIBCPP_INLINE_VISIBILITY
     void __append_default_init(size_type __n);
 
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
         >
     _LIBCPP_INLINE_VISIBILITY
     append(_InputIterator __first, _InputIterator __last) {
       const basic_string __temp(__first, __last, __alloc());
       append(__temp.data(), __temp.size());
       return *this;
     }
     template<class _ForwardIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             basic_string&
         >
     _LIBCPP_INLINE_VISIBILITY
     append(_ForwardIterator __first, _ForwardIterator __last);
 
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string& append(initializer_list<value_type> __il) {return append(__il.begin(), __il.size());}
 #endif // _LIBCPP_CXX03_LANG
 
     void push_back(value_type __c);
     _LIBCPP_INLINE_VISIBILITY
     void pop_back();
     _LIBCPP_INLINE_VISIBILITY reference       front() _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY const_reference front() const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY reference       back() _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY const_reference back() const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                  assign(const _Tp & __t) { __self_view __sv = __t; return assign(__sv.data(), __sv.size()); }
     _LIBCPP_INLINE_VISIBILITY
     basic_string& assign(const basic_string& __str) { return *this = __str; }
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string& assign(basic_string&& __str)
         _NOEXCEPT_((__noexcept_move_assign_container<_Allocator, __alloc_traits>::value))
         {*this = _VSTD::move(__str); return *this;}
 #endif
     basic_string& assign(const basic_string& __str, size_type __pos, size_type __n=npos);
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
             && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   assign(const _Tp & __t, size_type __pos, size_type __n=npos);
     basic_string& assign(const value_type* __s, size_type __n);
     basic_string& assign(const value_type* __s);
     basic_string& assign(size_type __n, value_type __c);
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
         >
         assign(_InputIterator __first, _InputIterator __last);
     template<class _ForwardIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             basic_string&
         >
         assign(_ForwardIterator __first, _ForwardIterator __last);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string& assign(initializer_list<value_type> __il) {return assign(__il.begin(), __il.size());}
 #endif // _LIBCPP_CXX03_LANG
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string& insert(size_type __pos1, const basic_string& __str);
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                  insert(size_type __pos1, const _Tp& __t)
     { __self_view __sv = __t; return insert(__pos1, __sv.data(), __sv.size()); }
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   insert(size_type __pos1, const _Tp& __t, size_type __pos2, size_type __n=npos);
     basic_string& insert(size_type __pos1, const basic_string& __str, size_type __pos2, size_type __n=npos);
     basic_string& insert(size_type __pos, const value_type* __s, size_type __n);
     basic_string& insert(size_type __pos, const value_type* __s);
     basic_string& insert(size_type __pos, size_type __n, value_type __c);
     iterator      insert(const_iterator __pos, value_type __c);
     _LIBCPP_INLINE_VISIBILITY
     iterator      insert(const_iterator __pos, size_type __n, value_type __c);
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             iterator
         >
         insert(const_iterator __pos, _InputIterator __first, _InputIterator __last);
     template<class _ForwardIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             iterator
         >
         insert(const_iterator __pos, _ForwardIterator __first, _ForwardIterator __last);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     iterator insert(const_iterator __pos, initializer_list<value_type> __il)
                     {return insert(__pos, __il.begin(), __il.end());}
 #endif // _LIBCPP_CXX03_LANG
 
     basic_string& erase(size_type __pos = 0, size_type __n = npos);
     _LIBCPP_INLINE_VISIBILITY
     iterator      erase(const_iterator __pos);
     _LIBCPP_INLINE_VISIBILITY
     iterator      erase(const_iterator __first, const_iterator __last);
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(size_type __pos1, size_type __n1, const basic_string& __str);
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                   replace(size_type __pos1, size_type __n1, const _Tp& __t) { __self_view __sv = __t; return replace(__pos1, __n1, __sv.data(), __sv.size()); }
     basic_string& replace(size_type __pos1, size_type __n1, const basic_string& __str, size_type __pos2, size_type __n2=npos);
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string>::value,
             basic_string&
         >
                   replace(size_type __pos1, size_type __n1, const _Tp& __t, size_type __pos2, size_type __n2=npos);
     basic_string& replace(size_type __pos, size_type __n1, const value_type* __s, size_type __n2);
     basic_string& replace(size_type __pos, size_type __n1, const value_type* __s);
     basic_string& replace(size_type __pos, size_type __n1, size_type __n2, value_type __c);
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(const_iterator __i1, const_iterator __i2, const basic_string& __str);
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             basic_string&
         >
                   replace(const_iterator __i1, const_iterator __i2, const _Tp& __t) { __self_view __sv = __t; return replace(__i1 - begin(), __i2 - __i1, __sv); }
 
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(const_iterator __i1, const_iterator __i2, const value_type* __s, size_type __n);
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(const_iterator __i1, const_iterator __i2, const value_type* __s);
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(const_iterator __i1, const_iterator __i2, size_type __n, value_type __c);
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
         >
         replace(const_iterator __i1, const_iterator __i2, _InputIterator __j1, _InputIterator __j2);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(const_iterator __i1, const_iterator __i2, initializer_list<value_type> __il)
#endif
     _LIBCPP_INLINE_VISIBILITY
     allocator_type get_allocator() const _NOEXCEPT {return __alloc();}
 
     _LIBCPP_INLINE_VISIBILITY
     size_type find(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT;
     size_type find(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find(const value_type* __s, size_type __pos = 0) const _NOEXCEPT;
     size_type find(value_type __c, size_type __pos = 0) const _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     size_type rfind(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               rfind(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT;
     size_type rfind(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type rfind(const value_type* __s, size_type __pos = npos) const _NOEXCEPT;
     size_type rfind(value_type __c, size_type __pos = npos) const _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     size_type find_first_of(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_first_of(const _Tp& __t, size_type __pos = 0) const _NOEXCEPT;
     size_type find_first_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_first_of(const value_type* __s, size_type __pos = 0) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_first_of(value_type __c, size_type __pos = 0) const _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     size_type find_last_of(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_last_of(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT;
     size_type find_last_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_last_of(const value_type* __s, size_type __pos = npos) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_last_of(value_type __c, size_type __pos = npos) const _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     size_type find_first_not_of(const basic_string& __str, size_type __pos = 0) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_first_not_of(const _Tp &__t, size_type __pos = 0) const _NOEXCEPT;
     size_type find_first_not_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_first_not_of(const value_type* __s, size_type __pos = 0) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_first_not_of(value_type __c, size_type __pos = 0) const _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     size_type find_last_not_of(const basic_string& __str, size_type __pos = npos) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             size_type
         >
               find_last_not_of(const _Tp& __t, size_type __pos = npos) const _NOEXCEPT;
     size_type find_last_not_of(const value_type* __s, size_type __pos, size_type __n) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_last_not_of(const value_type* __s, size_type __pos = npos) const _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     size_type find_last_not_of(value_type __c, size_type __pos = npos) const _NOEXCEPT;
 
     _LIBCPP_INLINE_VISIBILITY
     int compare(const basic_string& __str) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             int
         >
         compare(const _Tp &__t) const _NOEXCEPT;
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             int
         >
          compare(size_type __pos1, size_type __n1, const _Tp& __t) const;
 
     _LIBCPP_INLINE_VISIBILITY
     int compare(size_type __pos1, size_type __n1, const basic_string& __str) const;
     int compare(size_type __pos1, size_type __n1, const basic_string& __str, size_type __pos2, size_type __n2=npos) const;
 
     template <class _Tp>
     inline _LIBCPP_INLINE_VISIBILITY
-        _EnableIf
+        __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string>::value,
             int
         >
         compare(size_type __pos1, size_type __n1, const _Tp& __t, size_type __pos2, size_type __n2=npos) const;
     int compare(const value_type* __s) const _NOEXCEPT;
     int compare(size_type __pos1, size_type __n1, const value_type* __s) const;
     int compare(size_type __pos1, size_type __n1, const value_type* __s, size_type __n2) const;
#endif // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
     // want to allow inlining of the copy constructor. However, we don't want
     // to call the __init() functions as those are marked as inline which may
     // result in over-aggressive inlining by the compiler, where our aim is
     // to only inline the fast path code directly in the ctor.
     void __init_copy_ctor_external(const value_type* __s, size_type __sz);
 
     template <class _InputIterator>
     inline
-    _EnableIf
+    __enable_if_t
     <
         __is_exactly_cpp17_input_iterator<_InputIterator>::value
     >
     __init(_InputIterator __first, _InputIterator __last);
 
     template <class _ForwardIterator>
     inline
-    _EnableIf
+    __enable_if_t
     <
         __is_cpp17_forward_iterator<_ForwardIterator>::value
     >
     __init(_ForwardIterator __first, _ForwardIterator __last);
 
     void __grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                    size_type __n_copy,  size_type __n_del,     size_type __n_add = 0);
     void __grow_by_and_replace(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
basic_string<_CharT, _Traits, _Allocator>::basic_string(const _Tp & __t, const _Allocator& __a)
     __init(__sv.data(), __sv.size());
 #if _LIBCPP_DEBUG_LEVEL == 2
     __get_db()->__insert_c(this);
 #endif
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _InputIterator>
-_EnableIf
+__enable_if_t
 <
     __is_exactly_cpp17_input_iterator<_InputIterator>::value
 >
 basic_string<_CharT, _Traits, _Allocator>::__init(_InputIterator __first, _InputIterator __last)
 {
     __zero();
 #ifndef _LIBCPP_NO_EXCEPTIONS
     try
catch (...)
             __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
         throw;
     }
 #endif // _LIBCPP_NO_EXCEPTIONS
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _ForwardIterator>
-_EnableIf
+__enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value
 >
 basic_string<_CharT, _Traits, _Allocator>::__init(_ForwardIterator __first, _ForwardIterator __last)
 {
     size_type __sz = static_cast<size_type>(_VSTD::distance(__first, __last));
     if (__sz > max_size())
         this->__throw_length_error();
__move_assign(__str, integral_constant<bool,
           __alloc_traits::propagate_on_container_move_assignment::value>());
     return *this;
 }
 
 #endif
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
-_EnableIf
+__enable_if_t
 <
      __is_exactly_cpp17_input_iterator<_InputIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::assign(_InputIterator __first, _InputIterator __last)
 {
     const basic_string __temp(__first, __last, __alloc());
     assign(__temp.data(), __temp.size());
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
-_EnableIf
+__enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::assign(_ForwardIterator __first, _ForwardIterator __last)
 {
     size_type __cap = capacity();
     size_type __n = __string_is_trivial_iterator<_ForwardIterator>::value ?
basic_string<_CharT, _Traits, _Allocator>::assign(const basic_string& __str, size_type __pos, size_type __n)
     size_type __sz = __str.size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     return assign(__str.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
     && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::assign(const _Tp & __t, size_type __pos, size_type __n)
 {
     __self_view __sv = __t;
else
         __set_long_size(__sz+1);
     }
     traits_type::assign(*__p, __c);
     traits_type::assign(*++__p, value_type());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
-_EnableIf
+__enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::append(
     _ForwardIterator __first, _ForwardIterator __last)
 {
     size_type __sz = size();
basic_string<_CharT, _Traits, _Allocator>::append(const basic_string& __str, size_type __pos, size_type __n)
     size_type __sz = __str.size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     return append(__str.data() + __pos, _VSTD::min(__n, __sz - __pos));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-    _EnableIf
+    __enable_if_t
     <
         __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
         basic_string<_CharT, _Traits, _Allocator>&
     >
 basic_string<_CharT, _Traits, _Allocator>::append(const _Tp & __t, size_type __pos, size_type __n)
 {
     __self_view __sv = __t;
     size_type __sz = __sv.size();
if (__n)
         __set_size(__sz);
         traits_type::assign(__p[__sz], value_type());
     }
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
-_EnableIf
+__enable_if_t
 <
    __is_exactly_cpp17_input_iterator<_InputIterator>::value,
    typename basic_string<_CharT, _Traits, _Allocator>::iterator
 >
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, _InputIterator __first, _InputIterator __last)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__pos) == this,
         "string::insert(iterator, range) called with an iterator not"
         " referring to this string");
 #endif
     const basic_string __temp(__first, __last, __alloc());
     return insert(__pos, __temp.data(), __temp.data() + __temp.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _ForwardIterator>
-_EnableIf
+__enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::iterator
 >
 basic_string<_CharT, _Traits, _Allocator>::insert(const_iterator __pos, _ForwardIterator __first, _ForwardIterator __last)
 {
 #if _LIBCPP_DEBUG_LEVEL == 2
     _LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__pos) == this,
basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos1, const basic_string& __str,
     size_type __str_sz = __str.size();
     if (__pos2 > __str_sz)
         this->__throw_out_of_range();
     return insert(__pos1, __str.data() + __pos2, _VSTD::min(__n, __str_sz - __pos2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value  && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::insert(size_type __pos1, const _Tp& __t,
                                                   size_type __pos2, size_type __n)
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, size_type __n2, value_type __c)
     __set_size(__sz);
     __invalidate_iterators_past(__sz);
     traits_type::assign(__p[__sz], value_type());
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template<class _InputIterator>
-_EnableIf
+__enable_if_t
 <
     __is_cpp17_input_iterator<_InputIterator>::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::replace(const_iterator __i1, const_iterator __i2,
                                                    _InputIterator __j1, _InputIterator __j2)
 {
     const basic_string __temp(__j1, __j2, __alloc());
basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos1, size_type __n1, const basic_string& __str,
     size_type __str_sz = __str.size();
     if (__pos2 > __str_sz)
         this->__throw_out_of_range();
     return replace(__pos1, __n1, __str.data() + __pos2, _VSTD::min(__n2, __str_sz - __pos2));
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     basic_string<_CharT, _Traits, _Allocator>&
 >
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos1, size_type __n1, const _Tp& __t,
                                                    size_type __pos2, size_type __n2)
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::find(const basic_string& __str,
                                                 size_type __pos) const _NOEXCEPT
 {
     return __str_find<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find(const _Tp &__t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::rfind(const basic_string& __str,
                                                  size_type __pos) const _NOEXCEPT
 {
     return __str_rfind<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::rfind(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::find_first_of(const basic_string& __str,
                                                          size_type __pos) const _NOEXCEPT
 {
     return __str_find_first_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_first_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::find_last_of(const basic_string& __str,
                                                         size_type __pos) const _NOEXCEPT
 {
     return __str_find_last_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_last_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const basic_string& __str,
                                                              size_type __pos) const _NOEXCEPT
 {
     return __str_find_first_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_first_not_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const basic_string& __str,
                                                             size_type __pos) const _NOEXCEPT
 {
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __str.data(), __pos, __str.size());
 }
 
 template<class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     typename basic_string<_CharT, _Traits, _Allocator>::size_type
 >
 basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(const _Tp& __t,
                                                 size_type __pos) const _NOEXCEPT
 {
     __self_view __sv = __t;
basic_string<_CharT, _Traits, _Allocator>::find_last_not_of(value_type __c,
     return __str_find_last_not_of<value_type, size_type, traits_type, npos>
         (data(), size(), __c, __pos);
 }
 
 // compare
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     int
 >
 basic_string<_CharT, _Traits, _Allocator>::compare(const _Tp& __t) const _NOEXCEPT
 {
     __self_view __sv = __t;
     size_t __lhs_sz = size();
if (__r == 0)
         else if (__rlen > __n2)
             __r = 1;
     }
     return __r;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
     int
 >
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const _Tp& __t) const
 {
     __self_view __sv = __t;
     return compare(__pos1, __n1, __sv.data(), __sv.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 inline
 int
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const basic_string& __str) const
 {
     return compare(__pos1, __n1, __str.data(), __str.size());
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _Tp>
-_EnableIf
+__enable_if_t
 <
     __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value
     && !__is_same_uncvref<_Tp, basic_string<_CharT, _Traits, _Allocator> >::value,
     int
 >
 basic_string<_CharT, _Traits, _Allocator>::compare(size_type __pos1,
                                                    size_type __n1,
                                                    const _Tp& __t,

libcxx/include/tuple

public:
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY constexpr
         __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a)
             : __value_(__a)
         {static_assert(!is_reference<_Hp>::value,
               "Attempted to default construct a reference element in a tuple");}
 
     template <class _Tp,
-              class = _EnableIf<
+              class = __enable_if_t<
                   _And<
                       _IsNotSame<__uncvref_t<_Tp>, __tuple_leaf>,
                       is_constructible<_Hp, _Tp>
                     >::value
                 >
             >
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
         explicit __tuple_leaf(_Tp&& __t) _NOEXCEPT_((is_nothrow_constructible<_Hp, _Tp>::value))
template <class _Alloc>
             : _Hp(allocator_arg_t(), __a) {}
 
     template <class _Alloc>
         _LIBCPP_INLINE_VISIBILITY constexpr
         __tuple_leaf(integral_constant<int, 2>, const _Alloc& __a)
             : _Hp(__a) {}
 
     template <class _Tp,
-              class = _EnableIf<
+              class = __enable_if_t<
                   _And<
                     _IsNotSame<__uncvref_t<_Tp>, __tuple_leaf>,
                     is_constructible<_Hp, _Tp>
                   >::value
                 >
             >
         _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
         explicit __tuple_leaf(_Tp&& __t) _NOEXCEPT_((is_nothrow_constructible<_Hp, _Tp>::value))
class _LIBCPP_TEMPLATE_VIS tuple
     template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
         typename tuple_element<_Jp, tuple<_Up...> >::type&& get(tuple<_Up...>&&) _NOEXCEPT;
     template <size_t _Jp, class ..._Up> friend _LIBCPP_CONSTEXPR_AFTER_CXX11
         const typename tuple_element<_Jp, tuple<_Up...> >::type&& get(const tuple<_Up...>&&) _NOEXCEPT;
 public:
     // [tuple.cnstr]
 
     // tuple() constructors (including allocator_arg_t variants)
-    template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible, _EnableIf<
+    template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible, __enable_if_t<
         _And<
             _IsImpDefault<_Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
     tuple()
         _NOEXCEPT_(_And<is_nothrow_default_constructible<_Tp>...>::value)
     { }
 
     template <template<class...> class _IsImpDefault = __is_implicitly_default_constructible,
-              template<class...> class _IsDefault = is_default_constructible, _EnableIf<
+              template<class...> class _IsDefault = is_default_constructible, __enable_if_t<
         _And<
             _IsDefault<_Tp>...,
             _Not<_Lazy<_And, _IsImpDefault<_Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
     explicit tuple()
         _NOEXCEPT_(_And<is_nothrow_default_constructible<_Tp>...>::value)
     { }
 
-    template <class _Alloc, template<class...> class _IsImpDefault = __is_implicitly_default_constructible, _EnableIf<
+    template <class _Alloc, template<class...> class _IsImpDefault = __is_implicitly_default_constructible, __enable_if_t<
         _And<
             _IsImpDefault<_Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, _Alloc const& __a)
       : __base_(allocator_arg_t(), __a,
                     __tuple_indices<>(), __tuple_types<>(),
                     typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
                     __tuple_types<_Tp...>()) {}
 
     template <class _Alloc,
               template<class...> class _IsImpDefault = __is_implicitly_default_constructible,
-              template<class...> class _IsDefault = is_default_constructible, _EnableIf<
+              template<class...> class _IsDefault = is_default_constructible, __enable_if_t<
         _And<
             _IsDefault<_Tp>...,
             _Not<_Lazy<_And, _IsImpDefault<_Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, _Alloc const& __a)
       : __base_(allocator_arg_t(), __a,
                     __tuple_indices<>(), __tuple_types<>(),
                     typename __make_tuple_indices<sizeof...(_Tp), 0>::type(),
                     __tuple_types<_Tp...>()) {}
 
     // tuple(const T&...) constructors (including allocator_arg_t variants)
-    template <template<class...> class _And = _And, _EnableIf<
+    template <template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) >= 1>,
             is_copy_constructible<_Tp>...,
             is_convertible<const _Tp&, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     tuple(const _Tp& ... __t)
         _NOEXCEPT_(_And<is_nothrow_copy_constructible<_Tp>...>::value)
         : __base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                 typename __make_tuple_indices<0>::type(),
                 typename __make_tuple_types<tuple, 0>::type(),
                 __t...
                ) {}
 
-    template <template<class...> class _And = _And, _EnableIf<
+    template <template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) >= 1>,
             is_copy_constructible<_Tp>...,
             _Not<_Lazy<_And, is_convertible<const _Tp&, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(const _Tp& ... __t)
         _NOEXCEPT_(_And<is_nothrow_copy_constructible<_Tp>...>::value)
         : __base_(typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                 typename __make_tuple_indices<0>::type(),
                 typename __make_tuple_types<tuple, 0>::type(),
                 __t...
                ) {}
 
-    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) >= 1>,
             is_copy_constructible<_Tp>...,
             is_convertible<const _Tp&, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
         : __base_(allocator_arg_t(), __a,
                 typename __make_tuple_indices<sizeof...(_Tp)>::type(),
                 typename __make_tuple_types<tuple, sizeof...(_Tp)>::type(),
                 typename __make_tuple_indices<0>::type(),
                 typename __make_tuple_types<tuple, 0>::type(),
                 __t...
                ) {}
 
-    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) >= 1>,
             is_copy_constructible<_Tp>...,
             _Not<_Lazy<_And, is_convertible<const _Tp&, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, const _Alloc& __a, const _Tp& ... __t)
public:
 
     template <class ..._Up>
     struct _EnableUTypesCtor : _And<
         _BoolConstant<sizeof...(_Tp) >= 1>,
         _Not<_IsThisTuple<_Up...> >, // extension to allow mis-behaved user constructors
         is_constructible<_Tp, _Up>...
     > { };
 
-    template <class ..._Up, _EnableIf<
+    template <class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableUTypesCtor<_Up...>,
             is_convertible<_Up, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     tuple(_Up&&... __u)
         _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
         : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                     typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
-    template <class ..._Up, _EnableIf<
+    template <class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableUTypesCtor<_Up...>,
             _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(_Up&&... __u)
         _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
         : __base_(typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                     typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
-    template <class _Alloc, class ..._Up, _EnableIf<
+    template <class _Alloc, class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableUTypesCtor<_Up...>,
             is_convertible<_Up, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
         : __base_(allocator_arg_t(), __a,
                     typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                     typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
-    template <class _Alloc, class ..._Up, _EnableIf<
+    template <class _Alloc, class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableUTypesCtor<_Up...>,
             _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, const _Alloc& __a, _Up&&... __u)
         : __base_(allocator_arg_t(), __a,
                     typename __make_tuple_indices<sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Up)>::type(),
                     typename __make_tuple_indices<sizeof...(_Tp), sizeof...(_Up)>::type(),
                     typename __make_tuple_types<tuple, sizeof...(_Tp), sizeof...(_Up)>::type(),
                     _VSTD::forward<_Up>(__u)...) {}
 
     // Copy and move constructors (including the allocator_arg_t variants)
     tuple(const tuple&) = default;
     tuple(tuple&&) = default;
 
-    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
         _And<is_copy_constructible<_Tp>...>::value
     , int> = 0>
     tuple(allocator_arg_t, const _Alloc& __alloc, const tuple& __t)
         : __base_(allocator_arg_t(), __alloc, __t)
     { }
 
-    template <class _Alloc, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, template<class...> class _And = _And, __enable_if_t<
         _And<is_move_constructible<_Tp>...>::value
     , int> = 0>
     tuple(allocator_arg_t, const _Alloc& __alloc, tuple&& __t)
         : __base_(allocator_arg_t(), __alloc, _VSTD::move(__t))
     { }
 
     // tuple(const tuple<U...>&) constructors (including allocator_arg_t variants)
     template <class ..._Up>
     struct _EnableCopyFromOtherTuple : _And<
         _Not<is_same<tuple<_Tp...>, tuple<_Up...> > >,
         _Lazy<_Or,
             _BoolConstant<sizeof...(_Tp) != 1>,
             // _Tp and _Up are 1-element packs - the pack expansions look
             // weird to avoid tripping up the type traits in degenerate cases
             _Lazy<_And,
                 _Not<is_convertible<const tuple<_Up>&, _Tp> >...,
                 _Not<is_constructible<_Tp, const tuple<_Up>&> >...
             >
         >,
         is_constructible<_Tp, const _Up&>...
     > { };
 
-    template <class ..._Up, _EnableIf<
+    template <class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableCopyFromOtherTuple<_Up...>,
             is_convertible<const _Up&, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     tuple(const tuple<_Up...>& __t)
         _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, const _Up&>...>::value))
         : __base_(__t)
     { }
 
-    template <class ..._Up, _EnableIf<
+    template <class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableCopyFromOtherTuple<_Up...>,
             _Not<_Lazy<_And, is_convertible<const _Up&, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(const tuple<_Up...>& __t)
         _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, const _Up&>...>::value))
         : __base_(__t)
     { }
 
-    template <class ..._Up, class _Alloc, _EnableIf<
+    template <class ..._Up, class _Alloc, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableCopyFromOtherTuple<_Up...>,
             is_convertible<const _Up&, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, const _Alloc& __a, const tuple<_Up...>& __t)
         : __base_(allocator_arg_t(), __a, __t)
     { }
 
-    template <class ..._Up, class _Alloc, _EnableIf<
+    template <class ..._Up, class _Alloc, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableCopyFromOtherTuple<_Up...>,
             _Not<_Lazy<_And, is_convertible<const _Up&, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, const _Alloc& __a, const tuple<_Up...>& __t)
struct _EnableMoveFromOtherTuple : _And<
             _Lazy<_And,
                 _Not<is_convertible<tuple<_Up>, _Tp> >...,
                 _Not<is_constructible<_Tp, tuple<_Up> > >...
             >
         >,
         is_constructible<_Tp, _Up>...
     > { };
 
-    template <class ..._Up, _EnableIf<
+    template <class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableMoveFromOtherTuple<_Up...>,
             is_convertible<_Up, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     tuple(tuple<_Up...>&& __t)
         _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
         : __base_(_VSTD::move(__t))
     { }
 
-    template <class ..._Up, _EnableIf<
+    template <class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableMoveFromOtherTuple<_Up...>,
             _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(tuple<_Up...>&& __t)
         _NOEXCEPT_((_And<is_nothrow_constructible<_Tp, _Up>...>::value))
         : __base_(_VSTD::move(__t))
     { }
 
-    template <class _Alloc, class ..._Up, _EnableIf<
+    template <class _Alloc, class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableMoveFromOtherTuple<_Up...>,
             is_convertible<_Up, _Tp>... // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, const _Alloc& __a, tuple<_Up...>&& __t)
         : __base_(allocator_arg_t(), __a, _VSTD::move(__t))
     { }
 
-    template <class _Alloc, class ..._Up, _EnableIf<
+    template <class _Alloc, class ..._Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Up) == sizeof...(_Tp)>,
             _EnableMoveFromOtherTuple<_Up...>,
             _Not<_Lazy<_And, is_convertible<_Up, _Tp>...> > // explicit check
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, const _Alloc& __a, tuple<_Up...>&& __t)
public:
     template <class _Up1, class _Up2, class ..._DependentTp>
     struct _EnableExplicitCopyFromPair : _And<
         is_constructible<_FirstType<_DependentTp...>, const _Up1&>,
         is_constructible<_SecondType<_DependentTp...>, const _Up2&>,
         _Not<is_convertible<const _Up1&, _FirstType<_DependentTp...> > >, // explicit check
         _Not<is_convertible<const _Up2&, _SecondType<_DependentTp...> > >
     > { };
 
-    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableImplicitCopyFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     tuple(const pair<_Up1, _Up2>& __p)
         _NOEXCEPT_((_And<
             is_nothrow_constructible<_FirstType<_Tp...>, const _Up1&>,
             is_nothrow_constructible<_SecondType<_Tp...>, const _Up2&>
         >::value))
         : __base_(__p)
     { }
 
-    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableExplicitCopyFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(const pair<_Up1, _Up2>& __p)
         _NOEXCEPT_((_And<
             is_nothrow_constructible<_FirstType<_Tp...>, const _Up1&>,
             is_nothrow_constructible<_SecondType<_Tp...>, const _Up2&>
         >::value))
         : __base_(__p)
     { }
 
-    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableImplicitCopyFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, const _Alloc& __a, const pair<_Up1, _Up2>& __p)
         : __base_(allocator_arg_t(), __a, __p)
     { }
 
-    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableExplicitCopyFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, const _Alloc& __a, const pair<_Up1, _Up2>& __p)
         : __base_(allocator_arg_t(), __a, __p)
public:
     template <class _Up1, class _Up2, class ..._DependentTp>
     struct _EnableExplicitMoveFromPair : _And<
         is_constructible<_FirstType<_DependentTp...>, _Up1>,
         is_constructible<_SecondType<_DependentTp...>, _Up2>,
         _Not<is_convertible<_Up1, _FirstType<_DependentTp...> > >, // explicit check
         _Not<is_convertible<_Up2, _SecondType<_DependentTp...> > >
     > { };
 
-    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableImplicitMoveFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     tuple(pair<_Up1, _Up2>&& __p)
         _NOEXCEPT_((_And<
             is_nothrow_constructible<_FirstType<_Tp...>, _Up1>,
             is_nothrow_constructible<_SecondType<_Tp...>, _Up2>
         >::value))
         : __base_(_VSTD::move(__p))
     { }
 
-    template <class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableExplicitMoveFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     explicit tuple(pair<_Up1, _Up2>&& __p)
         _NOEXCEPT_((_And<
             is_nothrow_constructible<_FirstType<_Tp...>, _Up1>,
             is_nothrow_constructible<_SecondType<_Tp...>, _Up2>
         >::value))
         : __base_(_VSTD::move(__p))
     { }
 
-    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableImplicitMoveFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple(allocator_arg_t, const _Alloc& __a, pair<_Up1, _Up2>&& __p)
         : __base_(allocator_arg_t(), __a, _VSTD::move(__p))
     { }
 
-    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, _EnableIf<
+    template <class _Alloc, class _Up1, class _Up2, template<class...> class _And = _And, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == 2>,
             _EnableExplicitMoveFromPair<_Up1, _Up2, _Tp...>
         >::value
     , int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     explicit tuple(allocator_arg_t, const _Alloc& __a, pair<_Up1, _Up2>&& __p)
         : __base_(allocator_arg_t(), __a, _VSTD::move(__p))
tuple& operator=(_If<_And<is_move_assignable<_Tp>...>::value, tuple, __nat>&& __tuple)
         _NOEXCEPT_((_And<is_nothrow_move_assignable<_Tp>...>::value))
     {
         _VSTD::__memberwise_forward_assign(*this, _VSTD::move(__tuple),
             __tuple_types<_Tp...>(),
             typename __make_tuple_indices<sizeof...(_Tp)>::type());
         return *this;
     }
 
-    template<class... _Up, _EnableIf<
+    template<class... _Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == sizeof...(_Up)>,
             is_assignable<_Tp&, _Up const&>...
         >::value
     ,int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple& operator=(tuple<_Up...> const& __tuple)
         _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up const&>...>::value))
     {
         _VSTD::__memberwise_copy_assign(*this, __tuple,
             typename __make_tuple_indices<sizeof...(_Tp)>::type());
         return *this;
     }
 
-    template<class... _Up, _EnableIf<
+    template<class... _Up, __enable_if_t<
         _And<
             _BoolConstant<sizeof...(_Tp) == sizeof...(_Up)>,
             is_assignable<_Tp&, _Up>...
         >::value
     ,int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple& operator=(tuple<_Up...>&& __tuple)
         _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up>...>::value))
     {
         _VSTD::__memberwise_forward_assign(*this, _VSTD::move(__tuple),
             __tuple_types<_Up...>(),
             typename __make_tuple_indices<sizeof...(_Tp)>::type());
         return *this;
     }
 
-    template<class _Up1, class _Up2, class _Dep = true_type, _EnableIf<
+    template<class _Up1, class _Up2, class _Dep = true_type, __enable_if_t<
         _And<_Dep,
             _BoolConstant<sizeof...(_Tp) == 2>,
             is_assignable<_FirstType<_Tp..., _Dep>&, _Up1 const&>,
             is_assignable<_SecondType<_Tp..., _Dep>&, _Up2 const&>
         >::value
     ,int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple& operator=(pair<_Up1, _Up2> const& __pair)
         _NOEXCEPT_((_And<
             is_nothrow_assignable<_FirstType<_Tp...>&, _Up1 const&>,
             is_nothrow_assignable<_SecondType<_Tp...>&, _Up2 const&>
         >::value))
     {
         _VSTD::get<0>(*this) = __pair.first;
         _VSTD::get<1>(*this) = __pair.second;
         return *this;
     }
 
-    template<class _Up1, class _Up2, class _Dep = true_type, _EnableIf<
+    template<class _Up1, class _Up2, class _Dep = true_type, __enable_if_t<
         _And<_Dep,
             _BoolConstant<sizeof...(_Tp) == 2>,
             is_assignable<_FirstType<_Tp..., _Dep>&, _Up1>,
             is_assignable<_SecondType<_Tp..., _Dep>&, _Up2>
         >::value
     ,int> = 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple& operator=(pair<_Up1, _Up2>&& __pair)
         _NOEXCEPT_((_And<
             is_nothrow_assignable<_FirstType<_Tp...>&, _Up1>,
             is_nothrow_assignable<_SecondType<_Tp...>&, _Up2>
         >::value))
     {
         _VSTD::get<0>(*this) = _VSTD::forward<_Up1>(__pair.first);
         _VSTD::get<1>(*this) = _VSTD::forward<_Up2>(__pair.second);
         return *this;
     }
 
     // EXTENSION
-    template<class _Up, size_t _Np, class = _EnableIf<
+    template<class _Up, size_t _Np, class = __enable_if_t<
         _And<
             _BoolConstant<_Np == sizeof...(_Tp)>,
             is_assignable<_Tp&, _Up const&>...
         >::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple& operator=(array<_Up, _Np> const& __array)
         _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up const&>...>::value))
     {
         _VSTD::__memberwise_copy_assign(*this, __array,
             typename __make_tuple_indices<sizeof...(_Tp)>::type());
         return *this;
     }
 
     // EXTENSION
-    template<class _Up, size_t _Np, class = void, class = _EnableIf<
+    template<class _Up, size_t _Np, class = void, class = __enable_if_t<
         _And<
             _BoolConstant<_Np == sizeof...(_Tp)>,
             is_assignable<_Tp&, _Up>...
         >::value
     > >
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
     tuple& operator=(array<_Up, _Np>&& __array)
         _NOEXCEPT_((_And<is_nothrow_assignable<_Tp&, _Up>...>::value))

libcxx/include/type_traits

 #define _LIBCPP_BOOL_CONSTANT(__b) bool_constant<(__b)>
 #else
 #define _LIBCPP_BOOL_CONSTANT(__b) integral_constant<bool,(__b)>
 #endif
 
 template <bool, class _Tp = void> struct _LIBCPP_TEMPLATE_VIS enable_if {};
 template <class _Tp> struct _LIBCPP_TEMPLATE_VIS enable_if<true, _Tp> {typedef _Tp type;};
 
-template <bool _Bp, class _Tp = void> using _EnableIf _LIBCPP_NODEBUG = typename enable_if<_Bp, _Tp>::type;
+template <bool _Bp, class _Tp = void> using __enable_if_t _LIBCPP_NODEBUG = typename enable_if<_Bp, _Tp>::type;
 
 #if _LIBCPP_STD_VER > 11
 template <bool _Bp, class _Tp = void> using enable_if_t = typename enable_if<_Bp, _Tp>::type;
 #endif
 
 typedef _LIBCPP_BOOL_CONSTANT(true)  true_type;
 typedef _LIBCPP_BOOL_CONSTANT(false) false_type;
 

 struct _Not : _BoolConstant<!_Pred::value> {};
 template <class ..._Args>
 using _FirstType _LIBCPP_NODEBUG = typename _MetaBase<(sizeof...(_Args) >= 1)>::template _FirstImpl<_Args...>;
 template <class ..._Args>
 using _SecondType _LIBCPP_NODEBUG = typename _MetaBase<(sizeof...(_Args) >= 2)>::template _SecondImpl<_Args...>;
 
 template <class ...> using __expand_to_true = true_type;
 template <class ..._Pred>
-__expand_to_true<_EnableIf<_Pred::value>...> __and_helper(int);
+__expand_to_true<__enable_if_t<_Pred::value>...> __and_helper(int);
 template <class ...>
 false_type __and_helper(...);
 template <class ..._Pred>
 using _And _LIBCPP_NODEBUG = decltype(__and_helper<_Pred...>(0));
 
 template <template <class...> class _Func, class ..._Args>
 struct _Lazy : _Func<_Args...> {};
 
#ifdef __clang__
     !__is_same(_Tp, _Up)
 #else
     !is_same<_Tp, _Up>::value
 #endif
 >;
 
 
 template <class _Tp>
-using __test_for_primary_template = _EnableIf<
+using __test_for_primary_template = __enable_if_t<
     _IsSame<_Tp, typename _Tp::__primary_template>::value
   >;
 template <class _Tp>
 using __is_primary_template = _IsValidExpansion<
     __test_for_primary_template, _Tp
   >;
 
 struct __two {char __lx[2];};