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3486. is_constructible<T[], T...> may be misleading in C++20

Section: 20.15.5.4 [meta.unary.prop] Status: LEWG Submitter: Jonathan Wakely Opened: 2020-10-01 Last modified: 2020-10-04

Priority: Not Prioritized

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Discussion:

According to the current wording, std::is_constructible<int[], int> should be true, because the preconditions are met (all types are complete types or unbounded arrays) and the variable definition is well-formed since C++20:

using T = int[];
T t(declval<int>()); // equiv. to int t[] = {1};

However, this doesn't construct an object of type int[] because it deduces the array bound from the initializers, and so constructs int[1], which is not the type being asked about. It seems more logical for the trait to give a false result for an unbounded array, because it's an incomplete type, and no int[] can ever be constructed.

On the reflector Tim Song noted:

On the other hand, the result is something to which an int(&)[] can be bound directly thanks to another C++20 change, so a lot of things might Just Work (for some definition of "Work") despite the type difference.

This seems to me a reasonable rationale for is_constructible<int(&&)[], int> to be true (which it is today), but not for int[].

Peter Dimov replied:

Placement new, which is often the way to construct we're interested in, is not going to work even for T[2].

For example:

using T2 = int[2];
T2 x;
new(x) T2(1, 2); // ill-formed

We need to decide what behaviour we want here. Do we just want is_constructible to reflect the T(declval<Args...>); construct as currently specified in 20.15.5.4 [meta.unary.prop] p8, or do we want to give a more useful/meaningful answer here?

Should we revisit 20.15.5.4 [meta.unary.prop] p8 in the light of parenthesized aggregate init, so that is_constructible<T[], T> and is_constructible<T[1], T> are false?

There may be some interaction with LWG 3436.

Proposed resolution: