[Bug c++/112273] New: gcc crashs when checking satisfaction a constraint of lambda function

[Bug c++/112273] New: gcc crashs when checking satisfaction a constraint of lambda function

[rungecc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112273

            Bug ID: 112273
           Summary: gcc crashs when checking satisfaction a constraint of
                    lambda function
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rungecc at gmail dot com
  Target Milestone: ---

Created attachment 56468
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56468&action=edit
ICE output

```c++
template <class... Ts> struct Tp {};

template <class... T> struct make_delayed {
  static constexpr inline bool value = true;
};

template <class... T>
concept K = make_delayed<T...>::value;

[[maybe_unused]] constexpr auto ice(auto... arg) {
  using Tp_t = Tp<decltype(arg)...>;
  return []()
    requires K<Tp_t>
  { return 1; };
};

int main() {
  constexpr auto l = ice(1, 2);
  static_assert(l() == 1);
  return 0;
}
```

See also [online compiler](https://godbolt.org/z/r8K4bK199).

Gcc (checked version 14.0, 13.2, 12.3) with flag `-std=c++2b` crashed on the
above code snippets.

Output, see the attachment.

**NOTE**:

Seems we need 1) a dependent constraint attached to some function and 2) such
constraint refers to a type contains a (dependent) pack expansion in it's
definition to trigger this ICE.

[Bug c++/112273] gcc crashs when checking satisfaction a constraint of lambda function and using decltype of a variable argument template

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112273

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-10-29
            Summary|gcc crashs when checking    |gcc crashs when checking
                   |satisfaction a constraint   |satisfaction a constraint
                   |of lambda function          |of lambda function and
                   |                            |using decltype of a
                   |                            |variable argument template
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |c++-lambda
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, slightly reduced/changed:
```
template <class... Ts> struct Tp {};

template <class T> struct make_delayed {
  static constexpr inline bool value = true;
};

template <class T>
concept K = make_delayed<T>::value;


template<typename ...t>
constexpr auto ice(t... arg) {
  using Tp_t = Tp<decltype(arg)...>;
  return []() requires K<Tp_t>
  { return 1; };
};

int main() {
  constexpr auto l = ice(1);
  static_assert(l() == 1);
  return 0;
}
```

If I replace `decltype(arg)` with `t`, GCC no longer ICEs. This is why I moved
away from using auto here to show a case where it is working correctly.