Inference failure with -Ypartial-unification #13082
Description
Not sure if it's a bug, or if there are fundamental reasons why inference cannot succeed.
object Q {
def f[T, F[X <: T]](s: F[T])(implicit c: Co[T, F]) = ???
}
trait Itr[+A]
trait Co[U, -F[_ <: U]]
object Co {
implicit def i[E]: Co[E, Itr] = ???
}
class K
object K extends VS[K, VH[K]]
trait VS[V, H <: VH[V]] extends Itr[V]
trait VH[+A]
class Test {
// these all work with 2.12, but fail with 2.13
// they also fail with Scala 3, and with 2.12 with -Ypartial-unification
// inferred kinds of the type arguments (VH[K],[H <: VH[V]]VS[K,H]) do not conform to the expected kinds of the type parameters (type T,type F).
// [H <: VH[V]]VS[K,H]'s type parameters do not match type F's expected parameters:
// type H's bounds <: VH[V] are stricter than type X's declared bounds <: T
Q.f(K)
// Q.f(K)(Co.i)
// Q.f(K)(Co.i[K])
// Q.f(K)(Co.i[K])
// works on 2.12, 2.13, Scala 3
// Q.f[K, Itr](K)
}