[libcxx-commits] [libcxx] [libc++] Fix bug where optional<T&> couldn't be constructed from a monadic operation (PR #203462)

[A. Jiang via libcxx-commits]

================
@@ -776,6 +784,14 @@ template <class _Tp, class _Up, class... _Args>
 inline constexpr bool __is_constructible_for_optional_initializer_list_v<_Tp&, _Up, _Args...> = false;
 #    endif
 
+#    if _LIBCPP_STD_VER >= 26
+template <class _Tp>
+inline constexpr bool __is_valid_optional_type = is_object_v<_Tp> || is_lvalue_reference_v<_Tp>;
+#    else
+template <class _Tp>
+inline constexpr bool __is_valid_optional_type = is_object_v<_Tp>;
+#    endif
----------------
frederick-vs-ja wrote:

(Sorry for a possibly separated mail. This part isn't quite change-requested.)

Would it be clearer to only guard the portion added in C++26?
```suggestion
template <class _Tp>
inline constexpr bool __is_valid_optional_type =
#    if _LIBCPP_STD_VER >= 26
    is_lvalue_reference_v<_Tp> ||
#    endif
    is_object_v<_Tp>;
```

Also, would it be clearer to rename this to `__is_valid_optional_contained_type`?

https://github.com/llvm/llvm-project/pull/203462