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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113638] [13/14 Regression] Array bounds of variable
 templates are not correctly deduced from initializers since GCC13 inside a
 decltype/sizeof
Date: Sun, 28 Jan 2024 21:51:24 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 13.2.1
X-Bugzilla-Keywords: needs-bisection, wrong-code
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X-Bugzilla-Who: pinskia at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113638

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 =3D my_array<2>;
static_assert(sizeof(my_array<1>) =3D=3D 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...};
```=