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Subject: [Bug c++/102933] Can't use CTAD in template argument
Date: Wed, 15 Dec 2021 19:55:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102933

--- Comment #2 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:76c730cbf051281ff088ec18ee4f89fee4a8305a

commit r11-9387-g76c730cbf051281ff088ec18ee4f89fee4a8305a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Oct 28 10:46:46 2021 -0400

    c++: CTAD within template argument [PR102933]

    Here when checking for erroneous occurrences of 'auto' inside a template
    argument (which is allowed by the concepts TS for class templates),
    extract_autos_r picks up the CTAD placeholder for X{T{0}} which causes
    check_auto_in_tmpl_args to reject this valid template argument.  This
    patch fixes this by making extract_autos_r ignore CTAD placeholders.

    However, it seems we don't need to call check_auto_in_tmpl_args at all
    outside of the concepts TS since using 'auto' as a type-id is otherwise
    rejected more generally at parse time.  So this patch makes the function
    just exit early if !flag_concepts_ts.

    Similarly, I think the concepts code paths in do_auto_deduction and
    type_uses_auto are only necessary for the concepts TS, so this patch
    also restricts these code paths accordingly.

            PR c++/102933

    gcc/cp/ChangeLog:

            * parser.c (cp_parser_simple_type_specifier): Adjust diagnostic
            for using auto in parameter declaration.
            * pt.c (extract_autos_r): Ignore CTAD placeholders.
            (extract_autos): Use range-based for.
            (do_auto_deduction): Use extract_autos only for the concepts TS
            and not also for standard concepts.
            (type_uses_auto): Likewise with for_each_template_parm.
            (check_auto_in_tmpl_args): Just return false outside of the
            concepts TS.  Simplify.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/nontype-class50.C: New test.
            * g++.dg/cpp2a/nontype-class50a.C: New test.

    (cherry picked from commit f70f17d03630fd948bc15a646ebe7e784125c4d4)=