Can't implement Copy for C style enums with negative discriminants that overflow into unspecified positive discriminants

[mahkoh]

fn main() { }

#[repr(C)]
#[derive(Copy)]
enum X {
    A = -1,
    B,
}

test5.rs:7:5: 7:6 error: Discriminant overflowed!
test5.rs:7     B,
               ^

[huonw]

Providing an explicit discriminant for the variant after the -1 works as a workaround:

fn main() { }

#[repr(C)]
#[derive(Copy)]
enum X {
    A = -1,
    B = 0,
}

[mahkoh]

The same happens without repr(C) or with repr(i32).

[pnkfelix]

Also, -2 will work. I'll update the title.

[mahkoh]

@pnkfelix: The error message shows that it's about overflowing discriminants. There is nothing special about -1.

[pnkfelix]

@mahkoh my point is that the bug is not observed by all negative discriminants. But whatever, I'm not interested in fighting over how the title is phrased.

[mahkoh]

In any case the previous title was wrong because this works fine:

fn main() { }

//#[repr(C)]
enum _X {
    A = -1,
}

impl Copy for _X { }

[shepmaster]

@pnkfelix I think that @mahkoh's point was that this usage of -2 does fail:

fn main() { }

#[derive(Copy)]
enum X {
    A = -2,
    B,
    C,
}

[pnkfelix]

cc #23897