[Bug c++/105848] [12 Regression] undefined reference for default template argument of function type

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105848

--- Comment #11 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Patrick Palka
<[email protected]>:

https://gcc.gnu.org/g:9fad76fa86f5a1464fbe52f926750a389727910d

commit r16-9409-g9fad76fa86f5a1464fbe52f926750a389727910d
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]>
    (cherry picked from commit d47f48643b3db4e207b0cc3382f614aa76c9c2c5)