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

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

Differential Revision: https://reviews.llvm.org/D109435

NOKEYCHECK=True
GitOrigin-RevId: b4e88d4db12e9460e581de453c8603eb280f145b
diff --git a/include/string b/include/string
index f2606cb..8696bdf 100644
--- a/include/string
+++ b/include/string
@@ -832,7 +832,7 @@
     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");
@@ -842,7 +842,7 @@
 #   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);
 
@@ -857,7 +857,7 @@
     _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);
 
@@ -867,24 +867,24 @@
     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
@@ -901,7 +901,7 @@
 
     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);}
 
@@ -1004,7 +1004,7 @@
 
     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,
@@ -1022,7 +1022,7 @@
 
     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&
@@ -1032,7 +1032,7 @@
 
     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,
@@ -1048,7 +1048,7 @@
 
     template<class _InputIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_exactly_cpp17_input_iterator<_InputIterator>::value,
             basic_string&
@@ -1061,7 +1061,7 @@
     }
     template<class _ForwardIterator>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __is_cpp17_forward_iterator<_ForwardIterator>::value,
             basic_string&
@@ -1084,7 +1084,7 @@
 
     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&
@@ -1101,7 +1101,7 @@
     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,
@@ -1113,7 +1113,7 @@
     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&
@@ -1121,7 +1121,7 @@
         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&
@@ -1137,7 +1137,7 @@
 
     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&
@@ -1147,7 +1147,7 @@
 
     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&
@@ -1162,7 +1162,7 @@
     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
@@ -1170,7 +1170,7 @@
         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
@@ -1193,7 +1193,7 @@
 
     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&
@@ -1202,7 +1202,7 @@
     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&
@@ -1216,7 +1216,7 @@
 
     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&
@@ -1231,7 +1231,7 @@
     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&
@@ -1273,7 +1273,7 @@
 
     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
@@ -1289,7 +1289,7 @@
 
     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
@@ -1305,7 +1305,7 @@
 
     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
@@ -1322,7 +1322,7 @@
 
     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
@@ -1339,7 +1339,7 @@
 
     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
@@ -1356,7 +1356,7 @@
 
     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
@@ -1373,7 +1373,7 @@
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             int
@@ -1382,7 +1382,7 @@
 
     template <class _Tp>
     _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
-    _EnableIf
+    __enable_if_t
         <
             __can_be_converted_to_string_view<_CharT, _Traits, _Tp>::value,
             int
@@ -1395,7 +1395,7 @@
 
     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
@@ -1591,7 +1591,7 @@
 
     template <class _InputIterator>
     inline
-    _EnableIf
+    __enable_if_t
     <
         __is_exactly_cpp17_input_iterator<_InputIterator>::value
     >
@@ -1599,7 +1599,7 @@
 
     template <class _ForwardIterator>
     inline
-    _EnableIf
+    __enable_if_t
     <
         __is_cpp17_forward_iterator<_ForwardIterator>::value
     >
@@ -2136,7 +2136,7 @@
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _InputIterator>
-_EnableIf
+__enable_if_t
 <
     __is_exactly_cpp17_input_iterator<_InputIterator>::value
 >
@@ -2162,7 +2162,7 @@
 
 template <class _CharT, class _Traits, class _Allocator>
 template <class _ForwardIterator>
-_EnableIf
+__enable_if_t
 <
     __is_cpp17_forward_iterator<_ForwardIterator>::value
 >
@@ -2491,7 +2491,7 @@
 
 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>&
@@ -2505,7 +2505,7 @@
 
 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>&
@@ -2551,7 +2551,7 @@
 
 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,
@@ -2684,7 +2684,7 @@
 
 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>&
@@ -2737,7 +2737,7 @@
 
 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>&
@@ -2826,7 +2826,7 @@
 
 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
@@ -2844,7 +2844,7 @@
 
 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
@@ -2914,7 +2914,7 @@
 
 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>&
@@ -3071,7 +3071,7 @@
 
 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>&
@@ -3104,7 +3104,7 @@
 
 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>&
@@ -3563,7 +3563,7 @@
 
 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
@@ -3621,7 +3621,7 @@
 
 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
@@ -3679,7 +3679,7 @@
 
 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
@@ -3737,7 +3737,7 @@
 
 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
@@ -3795,7 +3795,7 @@
 
 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
@@ -3854,7 +3854,7 @@
 
 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
@@ -3892,7 +3892,7 @@
 
 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
@@ -3946,7 +3946,7 @@
 
 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
@@ -3971,7 +3971,7 @@
 
 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,