[committed] libstdc++: Fix value categories used by ranges access CPOs [PR 100824]
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 <firstname.lastname@example.org>
>> 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
>> (__member_size, __adl_size, __member_empty, __size0_empty):
>> (__eq_iter_empty, __adl_data): Use lvalue objects in
>> (__sentinel_size): Likewise. Add check for conversion to
>> (__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
>> * 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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