[Bug c++/95153] New: Arrays of 'const void *' should not be copyable

[Bug c++/95153] New: Arrays of 'const void *' should not be copyable

[alisdairm at me dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95153

            Bug ID: 95153
           Summary: Arrays of 'const void *' should not be copyable
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alisdairm at me dot com
  Target Milestone: ---

In general, we do not expect array types to be copyable according to the
'is_copy_constructible_v' trait.  However, the following started passing in gcc
10:

#include <type_traits>
static_assert(!std::is_copy_constructible_v<const void *[13]>);

Note that the 'const' is important, arrays of 'void *' remain non-copyable
according to the trait.

'void' may be a specific trigger too, as arrays of 'const int *' do not have
this problem.

Finally, the array bound does not seem significant, other than it gives the
expected answer again for an array of unknown bound.

[Bug c++/95153] Arrays of 'const void *' should not be copyable

[alisdairm at me dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95153

--- Comment #1 from Alisdair Meredith <alisdairm at me dot com> ---
Forgot to add this is specific to -std=c++20 too.

[Bug c++/95153] Arrays of 'const void *' should not be copyable in C++20

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95153

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #2 from TC <rs2740 at gmail dot com> ---
This is behaving as specified.

const void* const a[10];
const void* const b[10](a);

is valid code in C++20; it initializes b[0] with &a[0] converted to a const
void*, and the rest of b with null pointers.

The void* case is

void* const a[10];
void* const b[10](a);

which is invalid because &a[0] is a void* const*; converting that to void* is
not allowed because it casts away constness.

[Bug c++/95153] Arrays of 'const void *' should not be copyable in C++20

[alisdairm at me dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95153

--- Comment #3 from Alisdair Meredith <alisdairm at me dot com> ---
Sorry, thought I had confirmed that I agree with the analysis above - this is a
(perhaps surprising) change to the specification of C++20.

What is the best way to withdraw this report as invalid?

[Bug c++/95153] Arrays of 'const void *' should not be copyable in C++20

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95153

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Status -> RESOLVED INVALID

[Bug c++/95153] Arrays of 'const void *' should not be copyable in C++20

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95153

Barry Revzin <barry.revzin at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |barry.revzin at gmail dot com

--- Comment #5 from Barry Revzin <barry.revzin at gmail dot com> ---
Just to follow up, gcc trunk right now does this:

int main() {
    int a[3]{};
    void const* b[3](a);  // ok

    void const* c[3]{};
    void const* d[3](c);  // error: array must be initialized with a
brace-enclosed initializer
}

If 'd' is actually an error, then it's not because of that, so at the very
least, the diagnostic is wrong.