TASTY: no way to access baseClasses of AppliedType

[pshirshov]

I'm working on re-implementation of izumi-reflect for Dotty.

So far I made some substantial progress but seems like there is a blocker.

The primary problem is: seems like there is no way to get access to baseClasses of a type:

Let's assume that we have the following definitions:

trait X[A, B]
trait A[F[_]]
trait B extends A[X[Int, *]]

On scala2 I can call .baseClasses method on a type of B and get a list containing A[[T] => X[Int, T]].

In dotty AppliedType has only tycon method which does not allow me to inspect applied parents.

See also: 7mind/dotty-typetag-research#2 ; zio/izumi-reflect#22

[pshirshov]

@nicolasstucki could you check this issue please?

[Blaisorblade]

(On the main issue: I’m aware the API is incomplete and extensions are being considered, and things like this should be fine, but I cannot say more).

[jdegoes]

This seems like a useful improvement regardless, but speaking as a ZIO contributor, I know we are relying on this functionality (or its equivalent) for the new environment encoding, which has proven to be a smash hit among ZIO users, but which only has partial support in Dotty.

Thanks for reporting!

[pshirshov]

Seem like this is not enough. Seems like just this method does not allow to get applied supertype:

trait Super[X]
trait Sub extends Super[X]

I can get Super from .baseClasses output but not Super[X].

Seems like TypeApi#baseClass would solve my problem, but it's not exposed.
Could you expose it as well?

    /** The least type instance of given class which is a super-type
     *  of this type.  Example:
     *  {{{
     *    class D[T]
     *    class C extends p.D[Int]
     *    ThisType(C).baseType(D) = p.D[Int]
     * }}}
     */
    def baseType(clazz: Symbol): Type

[smarter]

If you need some API to be exposed, I encourage you to make a PR to do so yourself.

[pshirshov]

Yup, trying to do that.

[pshirshov]

@smarter : #9481 - what would you say? Just copycatted the original PR.