[Bug c++/91911] Strange interaction between CTAD and decltype

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91911

--- Comment #4 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:09845ad7569bac27c3a1dc7b410d9df764d2ca06

commit r12-6773-g09845ad7569bac27c3a1dc7b410d9df764d2ca06
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Jan 20 09:25:49 2022 -0500

    c++: CTAD inside alias template [PR91911, PR103672]

    In the first testcase below, when processing the alias template
    ConstSpanType, transparency of alias template specializations means we
    replace SpanType<T> with its instantiated definition.  But this
    instantiation lowers the level of the CTAD placeholder for span{T()} from
    2 to 1, and so the later instantiation of ConstSpanType<int> erroneously
    substitutes this CTAD placeholder with the template argument at level 1
    index 0, i.e. with int, before we get a chance to perform the CTAD.

    Although we represent CTAD placeholders as template parameters, we never
    actually want to replace them via tsubst.  So this patch adjusts tsubst
    to handle CTAD placeholders by simply substituting the template and
    returning a new CTAD placeholder.  Moreover, this means that the level
    of a CTAD placeholder doesn't matter, so we may as well give them all
    the same level.  This patch gives them the special level 0.

    The change in tsubst_decl removes a likely dead !CHECKING_P safeguard
    added in 2017, which would otherwise now get triggered for variables
    with CTAD placeholder types (since their level is 0).

            PR c++/91911
            PR c++/103672

    gcc/cp/ChangeLog:

            * pt.cc (keep_template_parm): Punt on a level 0 template parm.
            (tsubst_decl) <case VAR_DECL>: Remove !CHECKING_P safeguard.
            (tsubst) <case TEMPLATE_TYPE_PARM>: Handle CTAD placeholders
            specially.
            (make_auto_1): Add defaulted 'level' parameter.
            (make_template_placeholder): Pass 0 as 'level' to make_auto_1.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1z/class-deduction101.C: New test.
            * g++.dg/cpp1z/class-deduction101a.C: New test.
            * g++.dg/cpp1z/class-deduction101b.C: New test.
            * g++.dg/cpp1z/class-deduction102.C: New test.
            * g++.dg/cpp1z/class-deduction102a.C: New test.
            * g++.dg/cpp1z/class-deduction102b.C: New test.
            * g++.dg/cpp1z/class-deduction103.C: New test.