[libc++] P3379R1: Constrain 'std::expected' equality operators

Author: xiaoyang-sde

libcxx/include/__expected/expected.h

@@ -1140,11 +1140,15 @@ class expected : private __expected_base<_Tp, _Err> {
 
   // [expected.object.eq], equality operators
   template <class _T2, class _E2>
+#  if _LIBCPP_STD_VER >= 26
     requires(!is_void_v<_T2> &&
              requires(const _Tp& __tp, const _T2& __t2, const _Err& __err, const _E2& __e2) {
                { __tp == __t2 } -> convertible_to<bool>;
                { __err == __e2 } -> convertible_to<bool>;
              })
+#  else
+    requires(!is_void_v<_T2>)
+#  endif
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
@@ -1158,18 +1162,22 @@ class expected : private __expected_base<_Tp, _Err> {
   }
 
   template <class _T2>
+#  if _LIBCPP_STD_VER >= 26
     requires(!__is_std_expected<_T2>::value &&
              requires(const _Tp& __tp, const _T2& __t2) {
                { __tp == __t2 } -> convertible_to<bool>;
              })
+#  endif
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v) {
     return __x.__has_val() && static_cast<bool>(__x.__val() == __v);
   }
 
   template <class _E2>
+#  if _LIBCPP_STD_VER >= 26
     requires requires(const _Err& __err, const _E2& __e2) {
       { __err == __e2 } -> convertible_to<bool>;
     }
+#  endif
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e) {
     return !__x.__has_val() && static_cast<bool>(__x.__unex() == __e.error());
   }
@@ -1862,10 +1870,14 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
 
   // [expected.void.eq], equality operators
   template <class _T2, class _E2>
+#  if _LIBCPP_STD_VER >= 26
     requires(is_void_v<_T2> &&
              requires(const _Err& __err, const _E2& __e2) {
                { __err == __e2 } -> convertible_to<bool>;
              })
+#  else
+    requires is_void_v<_T2>
+#  endif
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
     if (__x.__has_val() != __y.__has_val()) {
       return false;
@@ -1875,9 +1887,11 @@ class expected<_Tp, _Err> : private __expected_void_base<_Err> {
   }
 
   template <class _E2>
+#  if _LIBCPP_STD_VER >= 26
     requires requires(const _Err& __err, const _E2& __e2) {
       { __err == __e2 } -> convertible_to<bool>;
     }
+#  endif
   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y) {
     return !__x.__has_val() && static_cast<bool>(__x.__unex() == __y.error());
   }