Spurious pattern exhaustivity warning when one type parameter lower-bounds another

This example:

sealed trait Parent
final case class Leaf[A, B >: A](a: A, b: B) extends Parent

def run(x: Parent): Unit = x match {
  case Leaf(a, b) =>
}

Leads to the following spurious warning:

-- [E029] Pattern Match Exhaustivity Warning:
6 |def run(x: Parent): Unit = x match {
  |                           ^
  |                           match may not be exhaustive.
  |
  |                           It would fail on pattern case: Leaf(_, _)
  |
  | longer explanation available when compiling with `-explain`
1 warning found

I suspect the issue to be different from #15522, since it is present on both main and 3.1.2.

[smarter]

note the asymmetry. To support F-bounds we only allow recursion in upper bounds.

Related: I realized recently that despite our best effort we could still get recursion in lower bounds: https://github.com/lampepfl/dotty/issues/15280 and I have no idea how to deal with that properly

[SethTisue]

Note that the original test case is fixable just with the frozen_<:< change.

So perhaps there’s yet another possible solution path here, namely:

Prevent the type avoidance crash (which only arises when we change the rhs of the pattern from () to a) by making type avoidance (or whatever relevant piece of machinery that type avoidance is employing) not choke on the cycle.