[libc++] Restore __synth_three_way lambda (#90398)

Restore `__synth_three_way` lambda to match the Standard. 
GH-57222 is done, restoring the Standard wording implementation should
be possible.


df28d4412c/libcxx/include/__compare/synth_three_way.h (L28)

According to comment
https://github.com/llvm/llvm-project/issues/59513#issuecomment-2068338762,
GH-59513 is not a blocker.

Co-authored-by: Hristo Hristov <zingam@outlook.com>

libcxx/include/__compare/synth_three_way.h

@@ -25,12 +25,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 // [expos.only.func]
 
-// TODO MODULES restore the lamba to match the Standard.
-// See https://github.com/llvm/llvm-project/issues/57222
-//_LIBCPP_HIDE_FROM_ABI inline constexpr auto __synth_three_way =
-//  []<class _Tp, class _Up>(const _Tp& __t, const _Up& __u)
-template <class _Tp, class _Up>
-_LIBCPP_HIDE_FROM_ABI constexpr auto __synth_three_way(const _Tp& __t, const _Up& __u)
+_LIBCPP_HIDE_FROM_ABI inline constexpr auto __synth_three_way = []<class _Tp, class _Up>(const _Tp& __t, const _Up& __u)
   requires requires {
     { __t < __u } -> __boolean_testable;
     { __u < __t } -> __boolean_testable;
@@ -45,7 +40,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto __synth_three_way(const _Tp& __t, const _Up
       return weak_ordering::greater;
     return weak_ordering::equivalent;
   }
-}
+};
 
 template <class _Tp, class _Up = _Tp>
 using __synth_three_way_result = decltype(std::__synth_three_way(std::declval<_Tp&>(), std::declval<_Up&>()));

libcxx/include/array

@@ -423,7 +423,7 @@ template <class _Tp, size_t _Size>
 _LIBCPP_HIDE_FROM_ABI constexpr __synth_three_way_result<_Tp>
 operator<=>(const array<_Tp, _Size>& __x, const array<_Tp, _Size>& __y) {
   return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Tp, _Tp>);
+      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // _LIBCPP_STD_VER <= 17

libcxx/include/deque

@@ -2531,7 +2531,7 @@ template <class _Tp, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<_Tp>
 operator<=>(const deque<_Tp, _Allocator>& __x, const deque<_Tp, _Allocator>& __y) {
   return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Tp, _Tp>);
+      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // _LIBCPP_STD_VER <= 17

libcxx/include/forward_list

@@ -1518,7 +1518,7 @@ template <class _Tp, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<_Tp>
 operator<=>(const forward_list<_Tp, _Allocator>& __x, const forward_list<_Tp, _Allocator>& __y) {
   return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Tp, _Tp>);
+      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // #if _LIBCPP_STD_VER <= 17

libcxx/include/list

@@ -1680,7 +1680,7 @@ template <class _Tp, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<_Tp>
 operator<=>(const list<_Tp, _Allocator>& __x, const list<_Tp, _Allocator>& __y) {
   return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Tp, _Tp>);
+      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // _LIBCPP_STD_VER <= 17

libcxx/include/map

@@ -1617,12 +1617,7 @@ operator<=(const map<_Key, _Tp, _Compare, _Allocator>& __x, const map<_Key, _Tp,
 template <class _Key, class _Tp, class _Compare, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<pair<const _Key, _Tp>>
 operator<=>(const map<_Key, _Tp, _Compare, _Allocator>& __x, const map<_Key, _Tp, _Compare, _Allocator>& __y) {
-  return std::lexicographical_compare_three_way(
-      __x.begin(),
-      __x.end(),
-      __y.begin(),
-      __y.end(),
-      std::__synth_three_way<pair<const _Key, _Tp>, pair<const _Key, _Tp>>);
+  return std::lexicographical_compare_three_way(__x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // #if _LIBCPP_STD_VER <= 17
@@ -2136,12 +2131,7 @@ template <class _Key, class _Tp, class _Compare, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<pair<const _Key, _Tp>>
 operator<=>(const multimap<_Key, _Tp, _Compare, _Allocator>& __x,
             const multimap<_Key, _Tp, _Compare, _Allocator>& __y) {
-  return std::lexicographical_compare_three_way(
-      __x.begin(),
-      __x.end(),
-      __y.begin(),
-      __y.end(),
-      std::__synth_three_way<pair<const _Key, _Tp>, pair<const _Key, _Tp>>);
+  return std::lexicographical_compare_three_way(__x.begin(), __x.end(), __y.begin(), __y.end(), __synth_three_way);
 }
 
 #endif // #if _LIBCPP_STD_VER <= 17

libcxx/include/set

@@ -993,8 +993,7 @@ operator<=(const set<_Key, _Compare, _Allocator>& __x, const set<_Key, _Compare,
 template <class _Key, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<_Key>
 operator<=>(const set<_Key, _Allocator>& __x, const set<_Key, _Allocator>& __y) {
-  return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Key, _Key>);
+  return std::lexicographical_compare_three_way(__x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // _LIBCPP_STD_VER <= 17
@@ -1454,7 +1453,7 @@ template <class _Key, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI __synth_three_way_result<_Key>
 operator<=>(const multiset<_Key, _Allocator>& __x, const multiset<_Key, _Allocator>& __y) {
   return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Key, _Key>);
+      __x.begin(), __x.end(), __y.begin(), __y.end(), __synth_three_way);
 }
 
 #endif // _LIBCPP_STD_VER <= 17

libcxx/include/vector

@@ -2903,7 +2903,7 @@ template <class _Tp, class _Allocator>
 _LIBCPP_HIDE_FROM_ABI constexpr __synth_three_way_result<_Tp>
 operator<=>(const vector<_Tp, _Allocator>& __x, const vector<_Tp, _Allocator>& __y) {
   return std::lexicographical_compare_three_way(
-      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way<_Tp, _Tp>);
+      __x.begin(), __x.end(), __y.begin(), __y.end(), std::__synth_three_way);
 }
 
 #endif // _LIBCPP_STD_VER <= 17