error[E0277]: the trait bound `for<'a> <_ as Yokeable<'a>>::Output: IsCovariant<'a>` is not satisfied --> $DIR/issue-90950.rs:50:12 | LL | upcast(y) | ------ ^ the trait `for<'a> IsCovariant<'a>` is not implemented for `<_ as Yokeable<'a>>::Output` | | | required by a bound introduced by this call | = help: the trait `IsCovariant<'a>` is implemented for `std::borrow::Cow<'a, T>` note: required by a bound in `upcast` --> $DIR/issue-90950.rs:27:42 | LL | fn upcast(x: Yoke) -> Yoke + 'static>> where | ------ required by a bound in this LL | Y: for<'a> Yokeable<'a>, LL | for<'a> >::Output: IsCovariant<'a> | ^^^^^^^^^^^^^^^ required by this bound in `upcast` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.