[Bug libstdc++/97120] New: circular concept loops in <ranges>

[Bug libstdc++/97120] New: circular concept loops in <ranges>

[avi@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97120

            Bug ID: 97120
           Summary: circular concept loops in <ranges>
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avi@cloudius-systems.com
  Target Milestone: ---

The following example compiles with gcc and fails with clang. I believe clang
is correct but can't prove it.

======= begin example =============
#include <ranges>

void foo() {
    std::ranges::iota_view iota(2, 10);
    iota.begin();
}
======== end example ==============

Clang complains that iota_view doesn't have a begin() member (which it does). I
think the reason is that the constraint is evaluated before iota_view is fully
defined. gcc evaluates lazily so it doesn't stumble on the problem.

The circular chain is:

view_interface<iota_view> is instantiated as a base class of iota_view. Clearly
iota_view isn't defined at this stage, it's just a forward-declared name.

view_interface instantiates iterator_t, which is an alias to
std::__detail::__range_iter_t.

__range_iter_t instantiates __ranges_begin.


__ranges_begin requires that its template parameter is a __member_begin<>
(among other options, but this is the valid one here).

__member_begin requires that the type (iota_view) have a begin() function. But
the type isn't defined yet.

I believe clang is correct, mostly because I believe concept evaluation should
be eager and not lazy, not because I know it for a fact.

Apologies for posting a clang issue here, I wouldn't if gcc could get
asan+coroutines working. The problem will hit gcc if it implements eager
evaluation too.

[Bug libstdc++/97120] circular concept loops in <ranges>

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97120

TC <rs2740 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rs2740 at gmail dot com

--- Comment #1 from TC <rs2740 at gmail dot com> ---
Concept evaluation is lazy, not eager. See [temp.inst]/18.

[Bug libstdc++/97120] circular concept loops in <ranges>

[avi@cloudius-systems.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97120

--- Comment #2 from Avi Kivity <avi@cloudius-systems.com> ---
This makes sense, according to my very limited ability to understand the
standard. I reflected it to the clang bug here:
https://bugs.llvm.org/show_bug.cgi?id=47509.

[Bug libstdc++/97120] circular concept loops in <ranges>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97120

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |MOVED
             Status|UNCONFIRMED                 |RESOLVED
           See Also|                            |https://bugs.llvm.org/show_
                   |                            |bug.cgi?id=47509

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a clang bug.