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

Jonathan Wakely jwakely@redhat.com
Fri Jun 4 20:46:08 GMT 2021

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
>>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>
>>	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.

Tested powerpc64le-linux. Committed to trunk.

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

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