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Subject: [Bug c++/98990] [10 Regression] ICE when two overloaded functions
 return `auto &&` and one accepts an `auto` parameter since
 r10-6571-ga6ee556c7659877b
Date: Wed, 31 Mar 2021 12:33:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98990

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:c76d503527394839f9192ee27abbc0626b4e40d8

commit r10-9637-gc76d503527394839f9192ee27abbc0626b4e40d8
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Feb 25 19:55:43 2021 -0500

    c++: abbreviated function template return type rewriting [PR98990]

    When an abbreviated function template has a complex placeholder return
    type such auto& or auto**, the level adjustment performed by
    splice_late_return_type directly replaces the 'auto' inside the original
    return type with the level-adjusted 'auto', but that breaks
    TYPE_CANONICAL caching.  Instead, we should rebuild the entire return
    type using the adjusted 'auto'.

    This patch makes this happen by tsubsting the original return type with
    an argument vector that maps the original 'auto' to the adjusted 'auto'.
    In passing, this patch also reverts the misguided changes to
    find_type_usage in r10-6571 that made find_type_usage return a tree*
    instead of a tree so as to discourage this kind of in-place type
    modification.

    It occurred to me that the constraint also needs to be rebuilt so that
    it refers to the adjusted 'auto', but this oversight doesn't seem to
    cause any issues at the moment due to how do_auto_deduction "manually"
    substitutes the 'auto' inside the constraint before performing
    satisfaction.  So this'll be fixed later as part of a rework of
    placeholder type constraint checking.

    gcc/cp/ChangeLog:

            PR c++/98990
            * pt.c (splice_late_return_type): Rebuild the entire return type
            if we have to adjust the level of an auto within.
            (type_uses_auto): Adjust call to find_type_usage.
            * type-utils.h (find_type_usage): Revert r10-6571 change that
            made this function return a pointer to the auto node.

    gcc/testsuite/ChangeLog:

            PR c++/98990
            * g++.dg/concepts/abbrev8.C: New test.

    (cherry picked from commit 6bd409cfc83683a9be5c6b3b8f9a3ec8959f9356)=