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From: "jlame646 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107065] GCC treats rvalue as an lvalue
Date: Wed, 28 Sep 2022 12:09:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107065

--- Comment #3 from Jason Liam <jlame646 at gmail dot com> ---
(In reply to Jonathan Wakely from comment #1)
> Reduced to remove the library dependency:
>=20
> enum Cat { prvalue, lvalue, xvalue };
>=20
> template<typename T>
> struct value_category {
>     // Or can be an integral or enum value
>     static constexpr auto value =3D prvalue;
> };
>=20
> template<typename T>
> struct value_category<T&> {
>     static constexpr auto value =3D lvalue;
> };
>=20
> template<typename T>
> struct value_category<T&&> {
>     static constexpr auto value =3D xvalue;
> };
>=20
> // Double parens for ensuring we inspect an expression,
> // not an entity
> #define VALUE_CATEGORY(expr) value_category<decltype((expr))>::value
>=20
> constexpr bool global =3D true;
> static_assert( VALUE_CATEGORY(!(!global)) =3D=3D prvalue, "GCC gets this =
right"
> );
>=20
> int main()
> {
>   bool b =3D true;
>   if ( VALUE_CATEGORY(!(!b)) !=3D prvalue )
>     throw 1;
> }

Here is another reduced demo: https://godbolt.org/z/hGhfrKrad

```
#include <iostream>
int main() {
    bool b =3D true;
    std::cout << std::is_same<decltype(!(!b)), bool>::value << "\n";

    auto bb =3D (!(!b));
    std::cout << std::is_same<decltype(bb), bool>::value << "\n";
}


```=