On 04/06/21 18:03 +0100, Jonathan Wakely wrote:
>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.

An additional problem with ranges::data was pointed out in the PR,
fixed with this patch.

Tested powerpc64le-linux. Committed to trunk.

This should also be backported to gcc-11 and gcc-10.