[Bug c++/115283] New: [14 Regression] "used but never defined" with extern templates

[Bug c++/115283] New: [14 Regression] "used but never defined" with extern templates

[gcc at hazardy dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

            Bug ID: 115283
           Summary: [14 Regression] "used but never defined" with extern
                    templates
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at hazardy dot de
  Target Milestone: ---

This reduced code does work with GCC 13, but with 14 I get the "used but never
defined" warning (and later undefined refernces, even tho the symbols are
emitted to the object files).

See https://godbolt.org/z/Gsz4x7d6h

#include <concepts>
#include <type_traits>

template<typename T>
class C
{
public:
    using ResultT = std::conditional_t<std::same_as<T, void>, T, T>;

    ResultT Result;

    C(ResultT result);
};

template<typename T>
C<T>::C(ResultT result)
    : Result{result}
{
    return;
}

extern template class C<bool>;

void foo()
{
    C<bool> r(true);
}

It is absolutely necessary to use the std::same_as for the conditional. If it
is replaced with true (or false) everything works. Even exchanging it with
sizeof(T) >= 2 works. std::is_same_v also works.

So maybe it's a bug in std::same_as?

[Bug c++/115283] [14 Regression] "used but never defined" with extern templates

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Started with r14-6789.

[Bug c++/115283] [14/15 Regression] "used but never defined" with extern templates

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.2
            Summary|[14 Regression] "used but   |[14/15 Regression] "used
                   |never defined" with extern  |but never defined" with
                   |templates                   |extern templates
           Keywords|                            |diagnostic

[Bug c++/115283] [14/15 Regression] "used but never defined" with extern templates

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 58311
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58311&action=edit
Reduced

[Bug c++/115283] [14/15 Regression] "used but never defined" with extern templates

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-05-29
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. attached the reduced testcase.

Note concept here for same is important, using `inline constexpr bool` instead
does not cause the warning to happen.

[Bug c++/115283] [14/15 Regression] "used but never defined" with extern templates

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:b1fe718cbe0c8883af89f52e0aad3ebf913683de

commit r15-1227-gb1fe718cbe0c8883af89f52e0aad3ebf913683de
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 12 20:05:05 2024 -0400

    c++: visibility wrt concept-id as targ [PR115283]

    Like with alias templates, it seems we don't maintain visibility flags
    for concepts either, so min_vis_expr_r should ignore them for now.
    Otherwise after r14-6789 we may incorrectly give a function template that
    uses a concept-id in its signature internal linkage.

            PR c++/115283

    gcc/cp/ChangeLog:

            * decl2.cc (min_vis_expr_r) <case TEMPLATE_DECL>: Ignore
            concepts.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/linkage5.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>

[Bug c++/115283] [14 Regression] "used but never defined" with extern templates

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[14/15 Regression] "used    |[14 Regression] "used but
                   |but never defined" with     |never defined" with extern
                   |extern templates            |templates
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
           Keywords|                            |link-failure
             Status|NEW                         |ASSIGNED

[Bug c++/115283] [14 Regression] "used but never defined" with extern templates

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-14 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:9583f781e17d4da881ee64db43af939402331413

commit r14-10318-g9583f781e17d4da881ee64db43af939402331413
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 12 20:05:05 2024 -0400

    c++: visibility wrt concept-id as targ [PR115283]

    Like with alias templates, it seems we don't maintain visibility flags
    for concepts either, so min_vis_expr_r should ignore them for now.
    Otherwise after r14-6789 we may incorrectly give a function template that
    uses a concept-id in its signature internal linkage.

            PR c++/115283

    gcc/cp/ChangeLog:

            * decl2.cc (min_vis_expr_r) <case TEMPLATE_DECL>: Ignore
            concepts.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/linkage5.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit b1fe718cbe0c8883af89f52e0aad3ebf913683de)

[Bug c++/115283] [14 Regression] "used but never defined" with extern templates

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115283

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

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

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 14.2, thanks for the bug report.