#![feature(transmutability)]
use std::mem::{Assume, MaybeUninit, TransmuteFrom};

pub fn is_transmutable<Src, Dst>() where Dst: TransmuteFrom<Src, { Assume::SAFETY }>{}

#[derive(Copy, Clone)]
#[repr(u8)]
enum B0 { Value = 0 }

#[derive(Copy, Clone)]
#[repr(u8)]
enum B1 { Value = 1 }

fn main() {
    is_transmutable::<(B0, B0), MaybeUninit<(B0, B0)>>(); // ok
    is_transmutable::<(B0, B0), MaybeUninit<(B0, B1)>>(); // unexpected error: `(B0, B0)` cannot be safely transmuted into `MaybeUninit<(B0, B1)>`
    is_transmutable::<(B0, B0), MaybeUninit<(B1, B0)>>(); // ok
    is_transmutable::<(B0, B0), MaybeUninit<(B1, B1)>>(); // ok
}