[Bug c++/113638] New: Array bounds of variable templates are not correctly deduced from initializers since GCC13

[Bug c++/113638] New: Array bounds of variable templates are not correctly deduced from initializers since GCC13

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

            Bug ID: 113638
           Summary: Array bounds of variable templates are not correctly
                    deduced from initializers since GCC13
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Keywords: accepts-invalid, rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

The following code snippet is incorrectly processed since C++13.

Godbolt link: https://godbolt.org/z/xjEqnqodj

```
#include <type_traits>

template<int ...Is>
constexpr int my_array[]{Is...};

static_assert(std::is_same_v<decltype(my_array<1, 2, 3, 4, 5>), const int[5]>);
// rejected since GCC13
static_assert(sizeof(my_array<1, 2, 3, 4, 5>) == sizeof(int) * 5); // rejected
since GCC13

static_assert(std::is_same_v<decltype(my_array<1, 2, 3, 4, 5>), const int[]>);
// passes since GCC13, but should not
```

It seems that GCC gives up deducing the array bounds from initializers of
variable templates since GCC13.

[Bug c++/113638] Array bounds of variable templates are not correctly deduced from initializers since GCC13

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

--- Comment #1 from Jiang An <de34 at live dot cn> ---
> The following code snippet is incorrectly processed since C++13.

Typo: this should be "since GCC13".

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.3
            Summary|Array bounds of variable    |[13/14 Regression] Array
                   |templates are not correctly |bounds of variable
                   |deduced from initializers   |templates are not correctly
                   |since GCC13                 |deduced from initializers
                   |                            |since GCC13

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-01-28
             Status|UNCONFIRMED                 |NEW

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

Simplified testcase:
```
template<int ...Is>
constexpr int my_array[]{Is...};
constexpr auto t1 = my_array<2>;
static_assert(sizeof(my_array<1>) == sizeof(int) * 1);
```

If we change the t1 variable definition to use `my_array<1>` instead of
`my_array<2>`, then the sizeof works.

It seems like we are not fully instantiating the template variable when used
inside a decltype/sizeof to figure out the array bounds.

An obvious workaround is to do:
```
template<int ...Is>
constexpr int my_array[sizeof...(Is)]{Is...};
```

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Started with r13-2540-g4db3cb781c3553.  If we remove the constexpr then it
seems we never accepted the testcase, so this is only a regression for
constexpr variable templates, but a proper fix will likely cover the
non-constexpr case too.

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
This seems to be the variable template version of
https://cplusplus.github.io/CWG/issues/408.html, the resolution of which
predated variable templates.

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #4)
> This seems to be the variable template version of
> https://cplusplus.github.io/CWG/issues/408.html, the resolution of which
> predated variable templates.

Note this works though (which I thought would be able the same, this is another
workaround):

```
template<int ...Is>
struct X
{
  static int s[];
};
template<int ...Is>
int X<Is...>::s[]={Is...};

static_assert(sizeof(X<1>::s) == sizeof(int) * 1, "");
```

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Indeed, looks like we implement CWG408 since r158075.

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

Jason Merrill <jason at gcc dot gnu.org> changed:

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

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:0b786ff38ab398087820d91241e030a28c451df9

commit r14-8726-g0b786ff38ab398087820d91241e030a28c451df9
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Feb 1 16:54:39 2024 -0500

    c++: variable template array of unknown bound [PR113638]

    When we added variable templates, we didn't extend the
VAR_HAD_UNKNOWN_BOUND
    handling for class template static data members to handle them as well.

            PR c++/113638

    gcc/cp/ChangeLog:

            * cp-tree.h: Adjust comment.
            * pt.cc (instantiate_template): Set VAR_HAD_UNKNOWN_BOUND for
            variable template.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/var-templ-array1.C: New test.

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jason Merrill
<jason@gcc.gnu.org>:

https://gcc.gnu.org/g:69f04177fed0de2c5f121b7607b7b59d3ba93451

commit r13-8272-g69f04177fed0de2c5f121b7607b7b59d3ba93451
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Feb 1 16:54:39 2024 -0500

    c++: variable template array of unknown bound [PR113638]

    When we added variable templates, we didn't extend the
VAR_HAD_UNKNOWN_BOUND
    handling for class template static data members to handle them as well.

            PR c++/113638

    gcc/cp/ChangeLog:

            * cp-tree.h: Adjust comment.
            * pt.cc (instantiate_template): Set VAR_HAD_UNKNOWN_BOUND for
            variable template.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/var-templ-array1.C: New test.

    (cherry picked from commit 0b786ff38ab398087820d91241e030a28c451df9)

[Bug c++/113638] [13/14 Regression] Array bounds of variable templates are not correctly deduced from initializers since GCC13 inside a decltype/sizeof

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113638

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #9 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for 13.3/14.