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

[Jonathan Wakely]

On 04/06/21 21:46 +0100, Jonathan Wakely wrote:
>On 04/06/21 21:44 +0100, Jonathan Wakely wrote:
>>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.
>
>And this implements the rest of LWG 3403. The change to the
>ranges::ssize constraints was already done by the first patch in this
>thread, this fixes the return type.

Aaaaand one more fix, as pointed out in the PR.

Tested x86_64-linux. Committed to trunk.

And this one should also be backported to gcc-11 and gcc-10.



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