Hello,

I was wondering if I can submit this improvement as a short proposal paper.

Discussion:

LWG3528 constraint `constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>)` with `requires is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...>`.

When someone write SFINAE code like below to check whether T can make from tuple, they may meet hard errors like "no matching function for call to 'make-from-tuple-impl'...".

```

template <class T, class Tuple, class = void>
inline constexpr bool has_make_from_tuple = false;

template <class T, class Tuple>
inline constexpr bool
    has_make_from_tuple<T, Tuple, std::void_t<decltype(std::make_from_tuple<T>(std::declval<Tuple>()))>> = true;

struct A { int a; };

static_assert(!has_make_from_tuple<int*, std::tuple<A*>>);

```

Proposed resolution:

This wording is relative to N4971.

Edit 22.4.6 [tuple.apply] as indicated:

template<class T, tuple-like Tuple>constexpr T make_from_tuple(Tuple&& t);-3- Let I be the pack 0, 1, ..., (tuple_size_v<remove_reference_t<Tuple>> - 1).-4- Constraints: is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...> is true.
-5- Mandates: If tuple_size_v<remove_reference_t<Tuple>> is 1, then reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))> is false.

-6- Effects: Given the exposition-only function template:
namespace std { template<class T, tuple-like Tuple, size_t... I> ~~requires is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...>~~ constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>) { // exposition only return T(get<I>(std::forward<Tuple>(t))...); } }
Equivalent to:return make-from-tuple-impl<T>( std::forward<Tuple>(t), make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});

[Note 1:
The type of T must be supplied as an explicit template parameter, as it cannot be deduced from the argument list.
— end note]

Implementation Experience:

I've implement this improvement in libc++ and STL.

https://github.com/llvm/llvm-project/pull/85263

https://github.com/microsoft/STL/pull/4528

Thanks,

Yrong