Why does std :: is_constructible stop in immediate context?

Question

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My question is: why does it std::is_constructiblestop in the immediate context? I think users std::is_constructiblewould expect it to work at full depth and give an accurate answer. With this immediate context, you can only std::is_constructiblegive you the green light, only to get a tough compiler error when you actually do it. Doesn't that defeat the original goal and goal std::is_constructible. Now it looks useless to me. I think it std::looks_constructible_at_first_sightwill be the best name for the current semantics :(

Answer 1

If your constructor signature is much more solvable than it should be, then the problem is not is_constructible. In the original example

template <typename... Ts, typename=decltype(base{std::declval<Ts>()...})>
aggregate_wrapper(Ts&&... xs)
    : base{std::forward<Ts>(xs)...} {/*…*/}

performs this work. If is_constructible"false" gives a green light, so your constructor template may be mistakenly selected over other constructors because the overload resolution finds the best match.

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