E4135

Inconsistent impl of trait: implementations have different self types or constraints.

When implementing different methods of a trait for the same type, all implementations must agree on the receiver type and type-parameter constraints. If two implementations use different constraints, it is unclear whether a type that satisfies both constraints has a complete and coherent implementation.

Erroneous example

pub(open) trait I {
  f(Self) -> Unit
  g(Self) -> Unit
}

pub(all) struct P[X](X)

impl[X : Show] I for P[X] with f(_self) {}
impl[X] I for P[X] with g(_self) {}

Suggestion

One way to resolve this issue is to make the implementations use the same constraints:

pub(open) trait I {
  f(Self) -> Unit
  g(Self) -> Unit
}

pub(all) struct P[X](X)

impl[X] I for P[X] with f(_self) {}
impl[X] I for P[X] with g(_self) {}

pub fn use_i(p : P[Int]) -> Unit {
  p.f()
  p.g()
}

If you want implementations for both constraints, create a new trait that combines the constraints, and manually implement that trait for each type that should satisfy it.