[Bug c++/110256] New: passing concept to concept compiles

[Bug c++/110256] New: passing concept to concept compiles

[ldalessandro at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110256

            Bug ID: 110256
           Summary: passing concept to concept compiles
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldalessandro at gmail dot com
  Target Milestone: ---

I accidentally passed a concept to convertible_to and was surprised to see it
compile.

```
#include <concepts>

template <class T>
concept b = requires (T t ) {
    { auto(t) } -> std::convertible_to<std::integral>;
};

static_assert(b<int>);
```

https://godbolt.org/z/jh53n185s

[Bug c++/110256] passing concept to concept compiles

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110256

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced to:
```
template<class T>
concept c = true;
template<class T, class T1>
concept  d = false;

template <class T>
concept b = requires (T t ) {
    { t } -> d<c>;
};

static_assert(!b<int>);
```

[Bug c++/110256] passing concept to concept compiles

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110256

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC also accepts this code ...
It might be the case that clang is incorrectly rejecting it.
Basically the way I understand is that b will always be false due to incorrect
usage of std::integral/c here. templates can be used as a template argument
even.

[Bug c++/110256] passing concept to concept compiles

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110256

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ppalka at gcc dot gnu.org
   Last reconfirmed|                            |2023-06-20
     Ever confirmed|0                           |1

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
We also incorrectly accept:

  template<class> concept c = true;
  template<class, class> concept d = true;
  d<c auto> auto x = 0;