[Bug c++/53164] Undefined reference to template function instantiation

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53164

--- Comment #7 from CVS 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:b5e38b1c166357e2a63d38ae6da7ae5d68fc115b

commit r13-6970-gb5e38b1c166357e2a63d38ae6da7ae5d68fc115b
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Apr 1 10:19:08 2023 -0400

    c++: improve "NTTP argument considered unused" fix [PR53164, PR105848]

    r13-995-g733a792a2b2e16 worked around the problem of (pointer to)
    function NTTP arguments not always getting marked as odr-used, by
    redundantly calling mark_used on the substituted ADDR_EXPR callee of a
    CALL_EXPR.  That is just a narrow workaround however, since it assumes
    the function is later called, but the use as a template argument alone
    should constitute an odr-use of the function (since template arguments
    are an evaluated context, and we're really passing its address); we
    shouldn't need to subsequently call or otherwise use the function NTTP
    argument.

    This patch fixes this in a more general way by walking the template
    arguments of each specialization that's about to be instantiated and
    redundantly calling mark_used on all entities used within.  As before,
    the call to mark_used as it worst a no-op, but it compensates for the
    situation where the specialization was first formed in a template context
    in which mark_used is inhibited.

    Another approach would be to call mark_used whenever we substitute a
    TEMPLATE_PARM_INDEX, but that would result in many more redundant calls
    to mark_used compared to this approach.  And as the second testcase
    below illustrates, we also need to walk C++20 class NTTP arguments which
    can be large and thus expensive to walk repeatedly.  The change to
    invalid_tparm_referent_p is needed to avoid incorrectly rejecting class
    NTTP arguments containing function pointers as in the testcase.

    (The third testcase is unrelated to this fix, but it helped rule out an
    earlier approach I was considering and it seems we don't have existing
    test coverage for this situation.)

            PR c++/53164
            PR c++/105848

    gcc/cp/ChangeLog:

            * pt.cc (invalid_tparm_referent_p): Accept ADDR_EXPR of
            FUNCTION_DECL.
            (instantiate_class_template): Call mark_template_arguments_used.
            (tsubst_copy_and_build) <case CALL_EXPR>: Revert r13-995 change.
            (mark_template_arguments_used): Define.
            (instantiate_body): Call mark_template_arguments_used.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/fn-ptr3a.C: New test.
            * g++.dg/template/fn-ptr3b.C: New test.
            * g++.dg/template/fn-ptr4.C: New test.