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From: "enrico.seiler at hotmail dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/100474] ICE: in diagnose_trait_expr, at cp/constraint.cc:3706
Date: Thu, 12 Aug 2021 07:47:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100474

Enrico Seiler <enrico.seiler at hotmail dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |enrico.seiler at hotmail d=
ot de

--- Comment #2 from Enrico Seiler <enrico.seiler at hotmail dot de> ---
Here is a version not dependent on range-v3:

https://godbolt.org/z/PvdhxGo5c

```
template <typename T, typename... Args>
concept foo =3D __is_constructible(T, Args...);

class S
{
public:
    S() =3D delete;
    S(S const &) =3D default;
    S(S &&) =3D default;
    S & operator=3D(S const &) =3D default;
    S & operator=3D(S &&) =3D default;
    ~S() =3D default;
};

int main()
{
    // static_assert(!__is_constructible(S)); // OK
    // static_assert(!foo<S>); // OK
    // static_assert(__is_constructible(S)); // FAILS
    static_assert(foo<S>); // ICE
}
```

The parameter pack is not necessary for the ICE to occur, however, it is wh=
at
META_IS_CONSTRUCTIBLE is expanded to in range-v3.

Using `concept foo =3D static_cast<bool>(__is_constructible(T, Args...));` =
will
remove the ICE, even though `__is_constructible` should already return a bo=
ol.=