Fix #9000: Avoid spurious cyclic errors for toplevel matches

[odersky]

I still don't know why the match type

type A2B[Xs <: Tuple] <: Tuple = Xs match
  case Unit => Unit
  case a *: as => B *: A2B[as]

compiled OK if it appeared in a user defined object, but failed with a cyclic
reference when written on the top level. But the logic for cyclic detection was
clearly very fragile, and unadapted to the case at hand. Essentially, it's a
type map that inserts LazyRefs when it detects legal cycles. The problem is that
these insertions cause recomputations of applied types in the TypeMap. And these
fail since the type constructor (A2B in this case) has not yet been completed.
So after a LazyRef was inserted, the cycle checker fails again in an unrecoverable
way because it tries to get the type parameters of an uncompleted type constructor.
So the question would be rather - why did this work if the match type appears in
a user-defined object? I could not answer that question, but I could fix the logic
to handle these cases.

[smarter]

So the question would be rather - why did this work if the match type appears in
a user-defined object?

Don't know exactly what's going on but: 1) in some situations we can complete type parameters without fully forcing the info using completerTypeParams and 2) appliedTo uses safeDealias to avoid forcing the type being applied, so there is some chance that an AppliedType can be mapped without causing a cycle I think.

[odersky]

I found the root cause. Here's the commit message of the last commit:

Fix #9000: Change definition of isTrivial

The only difference between checking cyclicity of a match type
in an object O and on the toplevel is that the matchtype itself
has the prefix O.this in the first case, and TermRef(_, pkgObject)
in the second case (this is because the match type is actually looked
up in the package scope, not in the package object scope).
This made a difference in the isTrivial check when computing type parameters.
And this in turn caused spurious cycles to be reported. The fix is
to add the missing case to isTrivial.