[Bug c++/97198] New: __is_constructible(int[], int) should return true

[Bug c++/97198] New: __is_constructible(int[], int) should return true

[kariya_mitsuru at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

            Bug ID: 97198
           Summary: __is_constructible(int[], int) should return true
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kariya_mitsuru at hotmail dot com
  Target Milestone: ---

The sample code below returns 0 if it is compiled by GCC HEAD with c++2a mode
but I think that it should return 1.

============ sample code ============
int main()
{
    return __is_constructible(int[], int);
}
============ sample code ============
cf. https://wandbox.org/permlink/nFQtqfxDNJPZRhZt


The C++20 DIS 20.15.4.3 Type properties[meta.unary.prop] p.8 says,

    The predicate condition for a template specialization is_constructible<T,
Args...> shall be satisfied if and only if the following variable definition
would be well-formed for some invented variable t:

    T t(declval<Args>()...);

    ...

cf. https://timsong-cpp.github.io/cppwp/n4861/meta#unary.prop-8


If T = int[] and Args = int, it is well-formed.

============ sample code ============
int main()
{
    using T = int[];
    T t(42);
}
============ sample code ============
cf. https://wandbox.org/permlink/2MhF1PNUbgq7mNAC

So, I think that __is_constructible(int[], int) should return true even though
int[] is imcomplete type. (unless it is a defect of the C++20 DIS)


Related PR:

PR94149 __is_constructible doesn't know about C++20 parenthesized init for
arrays
PR90532 [8/9/10 Regression] is_constructible_v<int[]> and
is_default_constructible_v<int[]> should agree

[Bug c++/97198] __is_constructible(int[], int) should return true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-09-25
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug c++/97198] __is_constructible(int[], int) should return true

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

[Bug c++/97198] __is_constructible(int[], int) should return true

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |redi at gcc dot gnu.org
             Status|ASSIGNED                    |SUSPENDED

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I'm actually not sure what we want to do.  I'd expect:

__is_constructible(int[]) // false in any mode
__is_constructible(int[], int) // true in C++20, false in C++17
__is_constructible(int[], int, int) // true in C++20, false in C++17

but that would go against PR90532.

Suspending until it's clear what we want.

[Bug c++/97198] __is_constructible(int[], int) should return true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Hmm. It should be false for construction from no arguments i.e.
__is_constructible(int[]).

But thanks to parenthesized aggregate init, you can actually do:

  using T = int[];
  T t(1);

It's still true that int[] is incomplete, but in the example above you actually
construct is a int[1] not int[]. I think this should be an LWG issue.

[Bug c++/97198] __is_constructible(int[], int) should return true

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> Hmm. It should be false for construction from no arguments i.e.
> __is_constructible(int[]).
> 
> But thanks to parenthesized aggregate init, you can actually do:
> 
>   using T = int[];
>   T t(1);
> 
> It's still true that int[] is incomplete, but in the example above you
> actually construct is a int[1] not int[].

Agreed.

> I think this should be an LWG issue.

Yes, I'd prefer LWG guidance before making any changes here.

[Bug c++/97198] __is_constructible(int[], int) should return true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is now https://wg21.link/lwg3486

[Bug c++/97198] __is_constructible(int[], int) should return true

[lichray at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

Zhihao Yuan <lichray at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lichray at gmail dot com

--- Comment #5 from Zhihao Yuan <lichray at gmail dot com> ---
Encountered this today. In case I cannot show up when discussing LWG3486, my
use case is that C(in_place_type<T[]>, a, b, c) should "just works." It's up to
C how to deal with it. In my case, it's new T[].

[Bug c++/97198] __is_constructible(int[], int) should return true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'll add a note to the lwg issue, as I'll never remember there was a comment
about it here.

[Bug c++/97198] __is_constructible(int[], int) should return true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Zhihao Yuan from comment #5)
> Encountered this today. In case I cannot show up when discussing LWG3486, my
> use case is that C(in_place_type<T[]>, a, b, c) should "just works." It's up
> to C how to deal with it. In my case, it's new T[].

I was going to add a note to the issue, but I don't know what to add. What is
C? Why wouldn't it work today? Why does std::is_constructible affect it?

[Bug c++/97198] __is_constructible(int[], int) should return true

[lichray at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97198

--- Comment #8 from Zhihao Yuan <lichray at gmail dot com> ---
(In reply to Jonathan Wakely from comment #7)
> (In reply to Zhihao Yuan from comment #5)
> > Encountered this today. In case I cannot show up when discussing LWG3486, my
> > use case is that C(in_place_type<T[]>, a, b, c) should "just works." It's up
> > to C how to deal with it. In my case, it's new T[].
> 
> I was going to add a note to the issue, but I don't know what to add. What
> is C? Why wouldn't it work today? Why does std::is_constructible affect it?

I meant to let C stand for some type C's constructor here. I wanted to express
the following: in_place_type is often used by type-erasures to forward all
information that is expression-equivalent to some form of initialization. Let's
say I want to create an object of type C with

  C obj(in_place_type<U>, a, b, c);

where obj erases the type of a hypothetical object created in

  U x(a, b, c);

Because type erasure means to erase the U in in_place_type<U>, it doesn't
matter if U != decltype(x). It works according to the literal interpretation of
the standard today. But if we say std::is_constructible_v<int[], int, int, int>
== false, extra efforts are needed to restore the meaning of in_place_type.