Conflicting implementations not overlapping

[tspiteri]

pub trait Cond {
    type Output;
}
pub struct S;
impl<T> From<T> for S where T: Cond<Output = i32> {}
impl<T> From<T> for S where T: Cond<Output = u32> {}

The implementations cannot overlap, because <T as Cond>::Output cannot be both i32 and u32. The code as far as I know does not use negative bounds which are problematic. That is because the second constraint isn't T: !Cond<Output = i32> or T: Cond<Output != i32>. Is there any reason I'm missing which would make accepting this a problem?

[RustyYato]

The workaround for this is a private trait

pub trait Cond {
    type Output;
}
pub struct S;

impl<T> From<T> for S where T: IntoS<T::Output> + Cond {
    fn from(value: T) -> Self { value.into_s() }
}

pub IntoS<C> {
    fn into_s(self) -> S;
}

impl<T: Cond<Output=u32>> IntoS<u32> for T {
    fn into_s(self) -> S { ... }
}

impl<T: Cond<Output=i32>> IntoS<i32> for T {
    fn into_s(self) -> S { ... }
}

But it would be nice if this workaround wasn't necessary.

[CAD97]

I believe this is issue#20400. RFC#1672 was proposed to fix this, but postponed for two main reasons. The first is that it's effectively blocked on chalk anyway, and the second is

@tspiteri

as far as I know does not use negative bounds which are problematic

that this isn't quite clear-cut, actually. As boats writes,

@withoutboats

I've realized this can express mutually exclusive traits and that it therefore carries all of the baggage associated with that feature.

// Two disjoint types
struct Left;
struct Right;

trait Exclusive {
    type Distinguisher;
}

// Foo and Bar are mutually exclusive because their associated types are
// disjoint.
trait Foo: Exclusive<Distinguisher = Left> { }
trait Bar: Exclusive<Distinguisher = Right> { }

trait Baz { }

// These impls are recognized as non-overlapping because the type parameters
// implement two exclusive forms of the same trait
impl<T: Foo> Baz for T { }
impl<T: Bar> Baz for T { }

[tspiteri]

Thanks for the workaround. Though in my case what I really wanted was From for the first condition and TryFrom for the second; so I created another trait instead of using TryFrom.