Dotty infers inexpressible type

Given:

trait Bippy 
trait Boppy extends Bippy
trait Foo[B <: Bippy, C] 
def bar[B <: Bippy, C](u1: Foo[_ <: B, C]): Foo[B, C] = ??? 
def foo: Foo[_ <: Boppy, _] = null

Evaluating the following in the REPL yields a method with an inferred type that cannot be expressed in user code.

scala> def baz = bar(foo) 
def baz: Foo[Boppy, foo.C]

foo is not a stable value.

scala> def baz: Foo[Boppy, foo.C] = ??? 
-- Error: <console>:8:20 -------------------------------------------------------
8 |def baz: Foo[Boppy, foo.C] = ???
  |                    ^^^
  |                    => Foo[_ <: Boppy, _](foo) is not stable

If I make foo a val, baz gets the same type but it is still not expressible.

scala> def baz: Foo[Boppy, foo.C] = ??? 
-- Error: <console>:8:24 -------------------------------------------------------
8 |def baz: Foo[Boppy, foo.C] = ???
  |                    ^^^^^
  |type C in class Foo cannot be accessed as a member of Foo[_ <: Boppy, _](foo) from module class $iw$.

I don’t know if this is caused by the REPL itself, because I don’t know of another way in which a mere mortal can currently inspect an inferred type in Dotty.

[smarter]

I don’t know of another way in which a mere mortal can currently inspect an inferred type in Dotty

Try compiling with -Xprint:frontend.

[Atry]

Why not simply allow a type that is path-dependent to a method?

Since the Dotty’s sanity check have been moved to the point of class instantiation, is stability check still necessary?