[Bug c++/126280] [13/14/15/16/17 Regression] "used but never defined" warning for function template used in variadic template from a template

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126280

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

https://gcc.gnu.org/g:d47f48643b3db4e207b0cc3382f614aa76c9c2c5

commit r17-2706-gd47f48643b3db4e207b0cc3382f614aa76c9c2c5
Author: Patrick Palka <[email protected]>
Date:   Sat Jul 25 16:12:19 2026 -0400

    c++: function NTTP argument in pack considered unused [PR126280]

    This is just the pack version of PR c++/105848, in which our non-dependent
    call pruning may cause us to not mark an otherwise unused function pointer
    template argument within a pack as used.

            PR c++/126280

    gcc/cp/ChangeLog:

            * pt.cc (mark_template_arguments_used): Split out loop into ...
            (mark_template_arguments_used_1): ... here. Recurse into
            argument packs.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/fn-ptr6.C: New test.

    Reviewed-by: Jason Merrill <[email protected]>