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From: "TonyELewis at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/96731] New: uniform_int_distribution requirement that
 its type is_integral is too strict
Date: Fri, 21 Aug 2020 06:19:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96731

            Bug ID: 96731
           Summary: uniform_int_distribution requirement that its type
                    is_integral is too strict
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: TonyELewis at hotmail dot com
  Target Milestone: ---

Following on from https://github.com/ericniebler/range-v3/issues/1532 ...

I think the static_assert requirement in libstdc++'s uniform_int_distributi=
on
code that its template argument type is_integral may be too strict.

I can't see any obvious mention of constraints on uniform_=C2=ADint_=C2=ADd=
istribution's
IntType in http://eel.is/c++draft/rand.dist.uni.int

The motivating case is attempting to sample from a range-v3 view::indices. =
The
range-v3 clever tricks lead to uniform_int_distribution being invoked with a
range-v3 type.

This all works OK under libc++ because it doesn't impose any requirements on
the type. But the aforementioned static_assert in libstdc++ won't allow it.

The problem can be seen with the first error under GCC or Clang with -std=
=3Dc++17
on this code:


#include <algorithm>
#include <iterator>
#include <random>
#include <vector>

#include <range/v3/view/indices.hpp>

void fill_vec_with_random_sample_of_first_n_ints( const size_t=20=20=20=20=
=20=20=20=20=20
&prm_num_possible_vals,
                                                  std::vector<size_t>
&prm_result,
                                                  std::mt19937        &prm_=
rng
                                                  ) {
        auto some_indices =3D ranges::views::indices( prm_num_possible_vals=
 );
        std::sample(
                std::begin( some_indices ),
                std::end  ( some_indices ),
                std::begin( prm_result ),
                static_cast<ptrdiff_t>( prm_result.size() ),
                prm_rng
        );
}


Compiler Explorer : https://godbolt.org/z/MxrazP


One option is that libstdc++ could use std::numeric_limits<T>::is_integer
instead of std::is_integral, so that range-v3 could then specialise accordi=
ngly
for its type.=