[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 via Gcc-bugs]

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.