Completions, higher-kinded types and match types

[kubukoz]

Describe the bug

Completions don't show up on members of a higher-kinded type if it's been applied with a match type (at least with non-standard types).

To Reproduce

Steps to reproduce the behavior:

1. Create a file with this content:

trait IO[A] {
  def map[B](f: A => B): IO[B]
}

case class User[F[_]](age: F[Int])

type M[A] = A match {
  case Int => IO[Int]
}

2. Now add:

def demo(x: M[A]) =
  x.age

and try to run completions on .age.

Expected behavior

Seeing map in the completions

Screenshots

Installation:

• Operating system: macOS
• Editor: Visual Studio Code
• Metals version: v0.10.3

Additional context

The methods show up if M[A] results in different types, but I'm under the impression we only see members of some supertypes. For example, List[Int]:

The foldLeft method on LinearSeq (the one I can navigate to) isn't deprecated - the one I see is probably from IterableOnceOps (that one is deprecated).

Search terms

match types, type matching, hkd, higher kinded data, completions

[tgodzik]

Thanks for reporting! I think match types are currently not that well supported in Dotty, we need to follow up in the compiler to see what's going on.

[tanishiking]

I confirmed this is fixed by scala/scala3#14639 in the latest nightly Scala3 version (3.1.3-RC2 or newer), thanks @rochala !
I'll add a test on Metals side