[Bug c++/102479] segfault when deducing class template arguments for tuple with libc++-14

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102479

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:f705d0f2d03cc4d9f890832798b3352553f2262f

commit r11-9992-gf705d0f2d03cc4d9f890832798b3352553f2262f
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Sep 27 16:01:10 2021 -0400

    c++: deduction guides and ttp rewriting [PR102479]

    The problem here is ultimately that rewrite_tparm_list when rewriting a
    TEMPLATE_TEMPLATE_PARM introduces a tree cycle in the rewritten
    ttp that structural_comptypes can't cope with.  In particular the
    DECL_TEMPLATE_PARMS of a ttp's TEMPLATE_DECL normally captures an empty
    parameter list at its own level (and so the TEMPLATE_DECL doesn't appear
    in its own DECL_TEMPLATE_PARMS), but rewrite_tparm_list ends up giving
    it a complete parameter list.  In the new testcase below, this causes
    infinite recursion from structural_comptypes when comparing Tmpl<char>
    with Tmpl<long> (where both 'Tmpl's are rewritten ttps).

    This patch fixes this by making rewrite_template_parm give a rewritten
    template template parm an empty parameter list at its own level, thereby
    avoiding the tree cycle.  Testing the alias CTAD case revealed that
    we're not setting current_template_parms in alias_ctad_tweaks, which
    this patch also fixes.

            PR c++/102479

    gcc/cp/ChangeLog:

            * pt.c (rewrite_template_parm): Handle single-level tsubst_args.
            Avoid a tree cycle when assigning the DECL_TEMPLATE_PARMS for a
            rewritten ttp.
            (alias_ctad_tweaks): Set current_template_parms accordingly.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/class-deduction12.C: Also test alias CTAD in the
            same way.
            * g++.dg/cpp1z/class-deduction99.C: New test.

    (cherry picked from commit 51018dd1395c72b3681ae5f84eceb94320472922)