[Bug libstdc++/100824] ranges::size should treat the subexpression as an lvalue

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100824

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:ee9548b36a7f17e8a63585b58f340c93dcba95d8

commit r12-1214-gee9548b36a7f17e8a63585b58f340c93dcba95d8
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Jun 4 15:59:38 2021 +0100

    libstdc++: Fix value categories used by ranges access CPOs [PR 100824]

    The implementation of P2091R0 was incomplete, so that some range access
    CPOs used perfect forwarding where they should not. This fixes it by
    consistently operating on lvalues.

    Some additional changes that are not necessary to fix the bug:

    Modify the __as_const helper to simplify its usage. Instead of deducing
    the value category from its argument, and requiring callers to forward
    the argument as the correct category, add a non-deduced template
    parameter which is used for the value category and accept the argument
    as an lvalue. This means callers say __as_const<T>(t) instead of
    __as_const(std::forward<T>(t)).

    Always use an lvalue reference type as the template argument for the
    _S_noexcept helpers, so that we only instantiate one specialization for
    lvalues and rvalues of the same type.

    Move some helper concepts and functions from namespace std::__detail
    to ranges::__cust_access, to be consistent with the ranges::begin CPO.
    This ensures that the __adl_begin concept and the _Begin::operator()
    function are in the same namespace, so unqualified lookup is consistent
    and the poison pills for begin are visible to both.

    Simplified static assertions for arrays, because the expression a+0 is
    already ill-formed for an array of incomplete type.

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

    libstdc++-v3/ChangeLog:

            PR libstdc++/100824
            * include/bits/iterator_concepts.h (__detail::__decay_copy)
            (__detail::__member_begin, __detail::__adl_begin): Move to
            namespace ranges::__cust_access.
            (__detail::__ranges_begin): Likewise, and rename to __begin.
            Remove redundant static assertion.
            * include/bits/ranges_base.h (_Begin, _End, _RBegin, _REnd):
            Use lvalue in noexcept specifier.
            (__as_const): Add non-deduced parameter for value category.
            (_CBegin, _CEnd, _CRBegin, _CREnd, _CData): Adjust uses of
            __as_const.
            (__member_size, __adl_size, __member_empty, __size0_empty):
            (__eq_iter_empty, __adl_data): Use lvalue objects in
            requirements.
            (__sentinel_size): Likewise. Add check for conversion to
            unsigned-like.
            (__member_data): Allow non-lvalue types to satisfy the concept,
            but use lvalue object in requirements.
            (_Size, _SSize): Remove forwarding to always use an lvalue.
            (_Data): Likewise. Add static assertion for arrays.
            * testsuite/std/ranges/access/cdata.cc: Adjust expected
            behaviour for rvalues. Add negative tests for ill-formed
            expressions.
            * testsuite/std/ranges/access/data.cc: Likewise.
            * testsuite/std/ranges/access/empty.cc: Adjust expected
            behaviour for rvalues.
            * testsuite/std/ranges/access/size.cc: Likewise.