* Allow Clang10 in the STL
This PR includes changes necessary to allow (but not require) clang 10 in the STL. It also includes test changes to allow the tests to pass given new clang warnings for deprecated behaviors, and an update to the LLVM reference to get similar changes that have been applied upstream to libc++ tests.
Details:
* In `<compare>`, remove workarounds for LLVM-41991 in Clang 10 RC1 fixed in RC2.
* In `<concepts>`, remove `_SILENCE_CLANG_CONCEPTS_MESSAGE`.
* In `<queue>` and `<stack>`, befriend only corresponding specializations of operator templates.
* In `<system_error>`, fix the `__cpp_constexpr_dynamic_alloc` implementation of `_Immortalize_memcpy_image` (which we apparently didn't review at all).
* In `<experimental/filesystem>`, apply a fix equivalent to the resolution of LWG-3244.
* Update `P0220R1_optional` from upstream.
* In `P0595R2_is_constant_evaluated`, silence Clang's warning for using `is_constant_evaluated` in a manifestly constant-evaluated context.
* In `P0896R4_ranges_iterator_machinery`, fix bogus test cases that were expecting VSO-1008447, silence "unused variable" warnings, and avoid taking advantage of too-lenient MSVC comparison rewrite behavior.
* In `P0896R4_ranges_range_machinery`, silence "unused variable" warning.
* In `P0898R3_concepts`, Remove workaround for LLVM-44627 in Clang 10 RC1 fixed in RC2.
* In `VSO_0000000_type_traits` and `tr1/type_traits5`, silence volatile function parameter deprecation warnings.
* In `tr1/condition_variable`, `tr1/regex1`, and `tr1/regex3`, remove unnecessary copy assignment operators that were prompting Clang warnings about the implicitly definition of a copy constructor for such a class being deprecated.
* In `tr1/csetjmp`, silence volatile increment deprecation warnings.
Skip new libc++ tests:
* Various `span` tests that expect `const_iterator` (libc++ doesn't yet implement LWG-3320)
* tests for the implementation of P1135R6 "The C++ Synchronization Library" which we do not yet implement
Includes:
* concepts `three_way_comparable` and `three_way_comparable_with`,
* type trait `std::compare_three_way_result` (with `_t` alias), and
* function object `compare_three_way`.
in `<compare>`, and:
* function objects `ranges::equal_to` and `ranges::less` (in `<xutility>` for
easy algorithm access), `ranges::not_equal_to`, `ranges::less_equal`,
`ranges::greater`, and `ranges::greater_equal` (in `<functional>`),
* slight refactoring of concept definitions in `<concepts>` to avoid redundant
requirements for the single-type comparison concepts `equality_comparable`,
`totally_ordered`, and `three_way_comparable`,
* heavy refactoring of the trait `common_comparison_category` (and `_t` alias)
in `<compare>` to requires only `n + c` template instantiations instead of `cn`
template instantiations,
* ======== ABI BREAK =========== remove the `_Is_unordered` member from
`std::partial_ordering` in `<compare>` since (a) it's true if and only if the
stored value has a particular value, and (b) Clang expects all of the
comparison category types to have size 1,
* reorder all `is_transparent` alias declarations in the STL to be after the
corresponding `operator()` to agree with synopsis order.
Also adds a new test `P0896R4_P1614R2_comparisons` that exercises the above.
The ABI BREAK sounds scary - as it should - but note that:
* this is a `/std:c++latest` feature, which is subject to change,
* objects of comparison category type aren't typically stored,
* only MSVC has released `<compare>` so far, so it's not widely used.
Altogether, it's extremely unlikely that anyone has encoded this in ABI.
I built clang from the `release/10.x` branch to investigate support for the portions of the STL that require concepts. This went swimmingly, until I was blocked by LLVM-44627 "Reversed candidate operator is not found by argument dependent lookup". These are a few fixes and workarounds I discovered in the process of getting some tests to pass before being blocked completely.
Detailed changes:
`<compare>`:
* Workaround LLVM-41991 "c++2a: attributes on defaulted friend functions incorrectly rejected" by using `_NODISCARD` on such functions only for non-clang.
`<concepts>`:
* Fix typo.
* Workaround LLVM-44689 "[concepts] ICE when *this appears in trailing requires-clause" in `std::ranges::swap`.
`<xhash>`:
* Silence clang warning about template parameter shadowing by renaming.
`<xutility>`:
* Clang thinks my `unreachable_sentinel_t` hack is ill-formed.
WG21-P1614 "The Mothership has Landed" added `==` operators to the comparison category types (`weak_equality`, `strong_equality`, `partial_ordering`, `weak_ordering`, and `strong_ordering`) defined in `<compare>`. This PR implements those operators to bring the comparison category types up to spec once again. It also implements P1614R2's removal of operators that rewrite into calls to `operator==(X, nullptr_t)` for each comparison category type `X`.
Drive-by changes:
* Move the TODO list of tasks for WG21-P0768 "Library Support for the Spaceship (Comparison) Operator" completion out of `<compare>` and into microsoft/STL#64
* Remove the `#if 0 // Not yet implemented` block from `<compare>`
Casey Carter
Adam Bucior
Stephan T. Lavavej
Daniel Marshall
Julie Philip James