Error when compiling ParSetLike, ParSet, SetLike, in this order

[smarter]

The following fails:

dotc -language:Scala2 scala-scala/src/library/scala/collection/parallel/mutable/ParSetLike.scala scala-scala/src/library/scala/collection/parallel/mutable/ParSet.scala scala-scala/src/library/scala/collection/mutable/SetLike.scala

-- Error: scala-scala/src/library/scala/collection/parallel/mutable/ParSetLike.scala 
34 |   with scala.collection.parallel.ParIterableLike[T, Repr, Sequential]
   |                                                           ^
   |Type argument Sequential does not conform to upper bound scala.collection.Iterable[T] & 
   |  scala.collection.IterableLike[T, LazyRef(Sequential)] 
-- Error: scala-scala/src/library/scala/collection/parallel/mutable/ParSetLike.scala 
35 |   with scala.collection.parallel.ParSetLike[T, Repr, Sequential]
   |                                                      ^
   |Type argument Sequential does not conform to upper bound scala.collection.Set[T] & scala.collection.SetLike[T, LazyRef(Sequential)] 

I've tried to trace what's going on and it looks a bit like this:

1. we first set the info of type Sequential to empty bounds in typeDefSig
2. Then we try to type its rhs
3. This forces a bunch of things, eventually we are again in typeDefSig and we try to type the rhs of type scala$collection$parallel$mutable$ParSetLike$$Sequential
4. type scala$collection$parallel$mutable$ParSetLike$$Sequential rhs is a derived type tree watching type Sequential, whose info is still set to empty bounds, so we get empty bounds here too, boom!

More precisely:

==> typing Sequential?
  ==> typing  <: mutable.Set[T] & mutable.SetLike[T, Sequential]?
    ==> typing mutable.Set[T] & mutable.SetLike[T, Sequential]?
       ==> typing mutable.Set[T]?
        ==> typing mutable.Set?
          ==> typing mutable?
             ==> adapting scala.collection.mutable of type scala.collection.mutable.type to ?{ Set: ? }?
               ==> typing Parallelizable[A, ParSet[A]]?
                 ==> typing ParSet[A]?
                   ==> typing ParSet?
                     ==> typing <derived typetree watching Sequential <: ...>?

I've come up with a slightly more minimal testcase, see https://gist.github.com/smarter/01643647f860df77b007b7c19dd013dd

[smarter]

A similar issue was fixed in 6c91684 by being careful about the order of completion. I don't know if we can do the same here.