[libcxx-commits] [libcxx] [libc++] Simplify the implementation of std::make_from_tuple (PR #215067)

[A. Jiang via libcxx-commits]

================
@@ -1362,79 +1362,56 @@ struct uses_allocator<tuple<_Tp...>, _Alloc> : true_type {};
 
 #    if _LIBCPP_STD_VER >= 17
 #      define _LIBCPP_NOEXCEPT_RETURN(...)                                                                             \
-        noexcept(noexcept(__VA_ARGS__)) { return __VA_ARGS__; }
+        noexcept(noexcept(__VA_ARGS__)) { return __VA_ARGS__; }                                                        \
+        static_assert(true)
 
-// The _LIBCPP_NOEXCEPT_RETURN macro breaks formatting.
-// clang-format off
 template <class _Fn, class _Tuple, size_t... _Id>
-inline _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto)
-__apply_tuple_impl(_Fn&& __f, _Tuple&& __t, index_sequence<_Id...>)
-    _LIBCPP_NOEXCEPT_RETURN(std::__invoke(std::forward<_Fn>(__f), std::get<_Id>(std::forward<_Tuple>(__t))...))
+inline
+    _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) __apply_tuple_impl(_Fn&& __f, _Tuple&& __t, index_sequence<_Id...>)
+        _LIBCPP_NOEXCEPT_RETURN(std::__invoke(std::forward<_Fn>(__f), std::get<_Id>(std::forward<_Tuple>(__t))...));
 
 template <class _Fn, class _Tuple>
 inline _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) apply(_Fn&& __f, _Tuple&& __t)
-    _LIBCPP_NOEXCEPT_RETURN(std::__apply_tuple_impl(
-        std::forward<_Fn>(__f),
-        std::forward<_Tuple>(__t),
-        make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>()))
+    _LIBCPP_NOEXCEPT_RETURN(std::__apply_tuple_impl(std::forward<_Fn>(__f),
+                                                    std::forward<_Tuple>(__t),
+                                                    make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>()));
+#      undef _LIBCPP_NOEXCEPT_RETURN
+
+template <class _Tp, class _Tuple, class _Seq = make_index_sequence<tuple_size_v<__remove_cvref_t<_Tuple>>>>
+inline constexpr bool __can_make_from_tuple_v = false;
 
-#if _LIBCPP_STD_VER >= 20
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frederick-vs-ja wrote:

I discussed with @timsong-cpp before and he confirmed that the it wasn't intended to add constraints to `make_from_tuple` in LWG3528. But some other implementors didn't seem to think so. I think it's fine to propagate the `requires` and translate the constraints into C++17 constructs. [LWG3628](https://cplusplus.github.io/LWG/issue3628) is a bit related.

Maybe @yronglin just wanted to "do what the standard says" in C++20 and thus only translated the resolution in C++17 mode. I'm fine to only use C++17 constructs if this makes the implementation details clearer.

https://github.com/llvm/llvm-project/pull/215067