Can't implement `From` or `Into` for custom wrapper type

[clarfonthey]

This won't work:

pub struct Wrapper<T>(T);
impl<T> From<Wrapper<T>> for T {
    fn from(val: Wrapper<T>) -> T { val.0 }
}

and neither will:

pub struct Wrapper<T>(T);
impl<T> Into<T> for Wrapper<T> {
    fn into(self) -> T { self.0 }
}

In previous (post-1.0) versions of the compiler, the latter was able to be implemented, however, now it's seen as conflicting with impl<T, U> Into<U> for T where U: From<T>, which is definitely the correct behaviour.

But right now, not being able to implement From for this case seems very much to me like a bug.

[durka]

Do you happen to know which version(s)? They both fail in 1.0: https://godbolt.org/g/Cay61k

The coherence issue with implementing From is that a downstream crate might want to implement From<Wrapper<TheirType>> for TheirType and they get priority.

[clarfonthey]

This should be fixed by #55437.

[leesongun]

It is still subject to the rule

2. No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)

and both doesn't work. (playground link)

[clarfonthey]

Right, I closed it but for the wrong reason. It's still not okay with the inference rules.