[Bug libstdc++/94971] [10/11 Regression] Parallel Mode cannot be used in C++20

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94971

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> This fails to compile in C++20 mode:
> 
> #define _GLIBCXX_PARALLEL 1
> #include <algorithm>
> 
> The new std::lexicographical_compare_three_way algo I added is in the wrong
> namespace.

This is fixed.

(In reply to Jonathan Wakely from comment #2)
> All the tests for C++20 constexpr algorithms fail, because the
> std::__parallel versions are not constexpr:
[...]
> I think that could be fixed by adding _GLIBCXX20_CONSTEXPR to them and then
> at the beginning of each function:
> 
> #if __cpp_lib_is_constant_evaluated
> if (std::is_constant_evaluated())
>   return _GLIBCXX_STD_A::replace_if(...);
> #endif

I don't plan to do that. 

> These fail to compile because the parallel algos don't support move
> semantics:
> FAIL: 25_algorithms/stable_sort/49559.cc (test for excess errors)
> FAIL: 25_algorithms/stable_sort/moveable.cc (test for excess errors)
> FAIL: 25_algorithms/stable_sort/moveable2.cc (test for excess errors)
> FAIL: 25_algorithms/partial_sort/moveable.cc (test for excess errors)
> FAIL: 25_algorithms/nth_element/moveable.cc (test for excess errors)
> FAIL: 25_algorithms/for_each/1.cc (test for excess errors)
> FAIL: 25_algorithms/for_each/for_each_n.cc (test for excess errors)

This has been broken for years.

> This fails because the new overload of std::search that takes a searcher
> isn't declared in std::__parallel:
> 
> FAIL: 25_algorithms/search/searcher.cc (test for excess errors)
> Excess errors:
> /home/jwakely/src/gcc/libstdc++-v3/testsuite/25_algorithms/search/searcher.
> cc:39: error: no matching function for call to 'search(int [2], int*,
> nocopy)'

Fixed.

> These fail at runtime, because the parallel algos copy instead of moving:
> 
> FAIL: 26_numerics/accumulate/lwg2055.cc execution test
> FAIL: 26_numerics/inner_product/lwg2055.cc execution test

Not going to be fixed.

I'm going to close this as WONTFIX. Parallel Mode should be considered legacy
code in maintenance mode only.