Issue with a nullary extension method and covariance

[flomebul]

minimized code

type Tr[+V, +O <: V]
def (tr: Tr[V, O]) sl[V, O <: V]: Tr[V, O] = ???
def as[V, O <: V](tr: Tr[V, O]) : Tr[V, O] = tr.sl

An error is found by the compiler whine none is expected

-- [E007] Type Mismatch Error: ABug.scala:5:45 ---------------------------------
5 |def as[V, O <: V] (tr: Tr[V, O]): Tr[V, O] = tr.sl
  |                                             ^^
  |                       Found:    queries.Tr[V, V]
  |                       Required: queries.Tr[V, O]
  |                       
  |                       where:    O is a type in method as with bounds <: V
  |                                 V is a type in method as with bounds >: O
  |                       
  |                       Constraint(
  |                        uninstVars = ;
  |                        constrained types = 
  |                        bounds = 
  |                        ordering = 
  |                       )
  |                       Subtype trace:

For information, the issue disappears if I replace nullary method by parameterless