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Subject: [Bug c++/101460] Useless cascade of overload resolution errors for
 invalid expression
Date: Mon, 19 Jul 2021 12:38:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101460

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:6dc150d9a036cbbed3c4dac6df1ce895b899d423

commit r11-8777-g6dc150d9a036cbbed3c4dac6df1ce895b899d423
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jul 14 20:14:14 2021 +0100

    libstdc++: Fix std::get<T> for std::tuple [PR101427]

    The std::get<T> functions relied on deduction failing if more than one
    base class existed for the type T.  However the implementation of Core
    DR 2303 (in r11-4693) made deduction succeed (and select the
    more-derived base class).

    This rewrites the implementation of std::get<T> to explicitly check for
    more than one occurrence of T in the tuple elements, making it
    ill-formed again. Additionally, the large wall of overload resolution
    errors described in PR c++/101460 is avoided by making std::get<T> use
    __get_helper<I> directly instead of calling std::get<I>, and by adding a
    deleted overload of __get_helper<N> for out-of-range N.

    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

    libstdc++-v3/ChangeLog:

            PR libstdc++/101427
            * include/std/tuple (tuple_element): Improve static_assert text.
            (__get_helper): Add deleted overload.
            (get<i>(tuple<T...>&&), get<i>(const tuple<T...>&&)): Use
            __get_helper directly.
            (__get_helper2): Remove.
            (__find_uniq_type_in_pack): New constexpr helper function.
            (get<T>): Use __find_uniq_type_in_pack and __get_helper instead
            of __get_helper2.
            * testsuite/20_util/tuple/element_access/get_neg.cc: Adjust
            expected errors.
            * testsuite/20_util/tuple/element_access/101427.cc: New test.

    (cherry picked from commit 17855eed7fc76b2cee7fbbc26f84d3c8b99be13c)=