[libcxx-commits] [libcxx] [llvm] [libc++] Implement P3379R0 Constrain `std::expected` equality operators (PR #135759)

[via libcxx-commits]

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@@ -1139,8 +1140,15 @@ class expected : private __expected_base<_Tp, _Err> {
 
   // [expected.object.eq], equality operators
   template <class _T2, class _E2>
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y)
     requires(!is_void_v<_T2>)
-  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
+#  if _LIBCPP_STD_VER >= 26
+            && requires {
+                 { *__x == *__y } -> __core_convertible_to<bool>;
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huixie90 wrote:

I would say it is very contrived and I would hardly believe the standard is designed to be in this form. IMO, implicit conversion should be a superset of explicit conversion. Implicitly conversion should imply explicit conversion. 

Having something is implicitly convertible to, but not explicitly convertible to , is , just , a foot gun. 

 This similar to non-const vs const. The example above is in the same spirit of having 

void f(const Foo&);
void f(Foo&) = delete;

I would say, why?

https://github.com/llvm/llvm-project/pull/135759