Bad type inference when type parameter as existential bound has invariant position #10740
Description
Compiles in dotty:
def diff1[A](xs: Set[A], ys: Set[_ <: A]) = xs -- ys
def diff2[A](xs: Set[_ >: A], ys: Set[A]) = xs -- ys
def diff3[A](xs: Set[_ >: A], ys: Set[_ <: A]) = xs -- ys
val xs = Set(Option(42))
val ys = Set(Some(42))
diff1(xs, ys)
diff2(xs, ys) // type mismatch: A is inferred as Option[Int]
diff3(xs, ys)Adapted from #5559 (comment)
Cause is similar to #10686 - when solving type variables the order of constraints matters.
Variables being solved here come from ExistentialType.withTypeVars.