Fix #2146: Make nested implicit function types work

[odersky]

Fix problems related to implicit function types.

1. Propagate expected type when typing an implicit closure.
   This is necessary to recursively apply eta expansion in the
   case of nested implicit functions.

2. Recursively transform definitions and accesses in shortcurImplicits

3. Generalize normalize to handle implicit function types

Review by @b-studios?

[odersky]

@felixmulder It fails because it does not find dotty.tools.dotc.ParallelTesting.

[b-studios]

@odersky Thanks alot for addressing the bug! Judging from the output of -Xprint:shortcutImplicits this looks really good to me. Finally we can use type aliases like:

type using[A, B] = implicit B => A
type and[A, B] = implicit B => A
def foo: Int using A and B = { ... }

Maybe add some tests on partially eta-expanded implicits?

Variant 1 - Outside-In (✔)

def foo: implicit A => implicit B => Int = { implicit a: A => ... }

Variant 2 - Inside-Out (✘)

def foo: implicit A => implicit B => Int = { implicit b: B => ... }

As I still know too little of the dotty compiler, I can't say much about how the code changes affect other existing functionality. You probably want to ask a second reviewer for that.

[Blaisorblade]

Thanks for dotty-staging#2.

As I still know too little of the dotty compiler, I can't say much about how the code changes affect other existing functionality. You probably want to ask a second reviewer for that.

👍. But FWIW, more recursion in normalize after taking a step is generally sensible.