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* [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict
@ 2020-08-21 6:19 TonyELewis at hotmail dot com
2020-08-21 8:52 ` [Bug libstdc++/96731] " redi at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: TonyELewis at hotmail dot com @ 2020-08-21 6:19 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96731
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_distribution
code that its template argument type is_integral may be too strict.
I can't see any obvious mention of constraints on uniform_int_distribution'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=c++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
&prm_num_possible_vals,
std::vector<size_t>
&prm_result,
std::mt19937 &prm_rng
) {
auto some_indices = 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 accordingly
for its type.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/96731] uniform_int_distribution requirement that its type is_integral is too strict
2020-08-21 6:19 [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict TonyELewis at hotmail dot com
@ 2020-08-21 8:52 ` redi at gcc dot gnu.org
2020-08-21 8:55 ` redi at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: redi at gcc dot gnu.org @ 2020-08-21 8:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96731
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Tony E Lewis from comment #0)
> I can't see any obvious mention of constraints on
> uniform_int_distribution's IntType in
> http://eel.is/c++draft/rand.dist.uni.int
See http://eel.is/c++draft/rand#req.genl-1.5
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/96731] uniform_int_distribution requirement that its type is_integral is too strict
2020-08-21 6:19 [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict TonyELewis at hotmail dot com
2020-08-21 8:52 ` [Bug libstdc++/96731] " redi at gcc dot gnu.org
@ 2020-08-21 8:55 ` redi at gcc dot gnu.org
2020-08-24 6:44 ` TonyELewis at hotmail dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: redi at gcc dot gnu.org @ 2020-08-21 8:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96731
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Severity|normal |enhancement
Last reconfirmed| |2020-08-21
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Since it's undefined, we can do whatever we want, including causing compilation
to fail with a static_assert, or ignoring it (as libc++ seems to do), or using
another condition to check for sufficiently integer-like types (possibly only
in non-strict modes).
Confirming as an enhancement request.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/96731] uniform_int_distribution requirement that its type is_integral is too strict
2020-08-21 6:19 [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict TonyELewis at hotmail dot com
2020-08-21 8:52 ` [Bug libstdc++/96731] " redi at gcc dot gnu.org
2020-08-21 8:55 ` redi at gcc dot gnu.org
@ 2020-08-24 6:44 ` TonyELewis at hotmail dot com
2020-08-26 19:15 ` redi at gcc dot gnu.org
2020-09-04 10:39 ` TonyELewis at hotmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: TonyELewis at hotmail dot com @ 2020-08-24 6:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96731
--- Comment #3 from Tony E Lewis <TonyELewis at hotmail dot com> ---
Thanks for correcting my failure to find the relevant part of the standard and
for confirming this as an enhancement request.
Yes please. If libstdc++'s sample() could play nicely with range-v3's
view::indices, that'd be much appreciated.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/96731] uniform_int_distribution requirement that its type is_integral is too strict
2020-08-21 6:19 [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict TonyELewis at hotmail dot com
` (2 preceding siblings ...)
2020-08-24 6:44 ` TonyELewis at hotmail dot com
@ 2020-08-26 19:15 ` redi at gcc dot gnu.org
2020-09-04 10:39 ` TonyELewis at hotmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: redi at gcc dot gnu.org @ 2020-08-26 19:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96731
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 49135
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49135&action=edit
Patch to relax std::uniform_int_distribution requirements
This patch allows a custom integer-like type to be used with
std::uniform_int_distribution, but it's harder than it seems.
We need to use std::make_unsigned, which is undefined for non-integral types.
The patch will use _IntType is numeric_limits<_IntType>::is_signed is false,
otherwise it will use make_unsigned_t<_IntType> if possible, and unsigned long
long otherwise.
But the example still fails, because std::sample also requires an integral
type, and also wants to use make_unsigned. And the standard is clear that it's
not just undefined, but ill-formed and requires a diagnostic:
> Mandates: For the overload in namespace std, Distance is an integer type
I don't plan to work on this any further for now.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug libstdc++/96731] uniform_int_distribution requirement that its type is_integral is too strict
2020-08-21 6:19 [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict TonyELewis at hotmail dot com
` (3 preceding siblings ...)
2020-08-26 19:15 ` redi at gcc dot gnu.org
@ 2020-09-04 10:39 ` TonyELewis at hotmail dot com
4 siblings, 0 replies; 6+ messages in thread
From: TonyELewis at hotmail dot com @ 2020-09-04 10:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96731
--- Comment #5 from Tony E Lewis <TonyELewis at hotmail dot com> ---
Thanks very much for your work on this.
That's a shame but I appreciate the problems you've highlighted.
> I don't plan to work on this any further for now.
Yes, fair enough.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2020-08-21 6:19 [Bug libstdc++/96731] New: uniform_int_distribution requirement that its type is_integral is too strict TonyELewis at hotmail dot com
2020-08-21 8:52 ` [Bug libstdc++/96731] " redi at gcc dot gnu.org
2020-08-21 8:55 ` redi at gcc dot gnu.org
2020-08-24 6:44 ` TonyELewis at hotmail dot com
2020-08-26 19:15 ` redi at gcc dot gnu.org
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