[Bug c++/104041] New: static_assert failure triggered by non-selected template specialization

[Bug c++/104041] New: static_assert failure triggered by non-selected template specialization

[bugzilla at cems dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104041

            Bug ID: 104041
           Summary: static_assert failure triggered by non-selected
                    template specialization
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugzilla at cems dot de
  Target Milestone: ---

A static_assert failure can be triggered even by a template specialization that
is not selected. 

The following code selects the specialization C<int> if allowed to compile, but
compilation fails if there is a failing static_assert in the generic template
C<T>, despite it not being selected.


#include <iostream>

template <typename T> struct C
{ C(T) {
#ifdef DEBUG
  static_assert(false, "C<T> selected");
#endif
  std::cout << "C<T> selected"; }
};

template <> struct C<int>
{ C(int) { std::cout << "C<int> selected"; }
};

int main()
{ C c(1);
}

/*
with macro DEBUG defined, compilation error with diagnostic:
error: static assertion failed: C<T> selected

without macro DEBUG defined, compiles and runs with output:
C<int> selected

*/

[Bug c++/104041] static_assert failure triggered by non-selected template specialization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104041

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Since the value that is done in the static_assert is not value dependent, it is
rejected at the definition time of C<T>::C<T>.

This is correct. 
Can you use either do the following instead:
template <typename T> struct C
{
    C(T) = delete;
};
Which requires a specialization for C<T>::C<T> so it will not be a deleted
constructor.

[Bug c++/104041] static_assert failure triggered by non-selected template specialization

[bugzilla at cems dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104041

bugzilla at cems dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |FIXED

--- Comment #2 from bugzilla at cems dot de ---
(In reply to Andrew Pinski from comment #1)
> Since the value that is done in the static_assert is not value dependent, it
> is rejected at the definition time of C<T>::C<T>.
> 
> This is correct. 

So if I replace the condition in the static_assert by something which is value
dependent,it will not be triggered in an unselected specialization even if the
condition is always false, e.g.:

static_assert(sizeof(T) < 0, "C<T> selected");

OK, thanks.

[Bug c++/104041] static_assert failure triggered by non-selected template specialization

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104041

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |INVALID

[Bug c++/104041] static_assert failure triggered by non-selected template specialization

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104041

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Please don't change the resolution to FIXED. There was no GCC bug, and so
nothing was fixed. The correct resolution is INVALID.

[Bug c++/104041] static_assert failure triggered by non-selected template specialization

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104041

--- Comment #4 from CVS 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:9944ca17c0766623bce260684edc614def7ea761

commit r13-6133-g9944ca17c0766623bce260684edc614def7ea761
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Feb 10 16:16:45 2023 -0800

    c++: static_assert (false) in template [DR2518]

    For a long time, people have expected to be able to write
    static_assert (false) in a template and only have it diagnosed if the
    template is instantiated, but we (and other implementations) gave an error
    about the uninstantiated template because the standard says that if no
valid
    instantiation of the template is possible, the program is ill-formed, no
    diagnostic required, and we try to diagnose IFNDR things when feasible.

    At the meeting last week we were looking at CWG2518, which wanted to
specify
    that an implementation must not accept a program containing a failing
#error
    or static_assert.  We also looked at P2593, which proposed allowing
    static_assert in an uninstantiated template.  We ended up combining these
    two in order to avoid requiring implementations to reject programs with
    static_assert (false) in uninstantiated templates.

    The committee accepted this as a DR, so I'm making the change to all
    standard modes.  This behavior was also conformant previously, since no
    diagnostic was required in this case.

    We continue to diagnose non-constant or otherwise ill-formed conditions, so
    no changes to existing tests were needed.

            DR 2518
            PR c++/52809
            PR c++/53638
            PR c++/87389
            PR c++/89741
            PR c++/92099
            PR c++/104041
            PR c++/104691

    gcc/cp/ChangeLog:

            * semantics.cc (finish_static_assert): Don't diagnose in
            template context.

    gcc/testsuite/ChangeLog:

            * g++.dg/DRs/dr2518.C: New test.