Improve Subtyping and Normalization of Capture Sets

[bracevac]

Compiler version

Latest nightly

Minimized code

As noted in PR #22000, some expected subtyping
relations are still not supported.

import language.experimental.captureChecking

abstract class Source[+T, Cap^]:
  def transformValuesWith[U](f: (T -> U)^{Cap^}): Source[U, Cap]^{this, f} = ???

def race[T, Cap^](sources: Source[T, Cap]^{Cap^}*): Source[T, Cap]^{Cap^} = ???

def either[T1, T2, Cap^](
    src1: Source[T1, Cap]^{Cap^},
    src2: Source[T2, Cap]^{Cap^}): Source[Either[T1, T2], Cap]^{Cap^} =
  val left = src1.transformValuesWith(Left(_))
  val right = src2.transformValuesWith(Right(_))
  race(left, right) // <- rejected
  //race[Either[T1, T2], Cap](left, right) // <- accepted

Expectation

The last line should not require explicit type arguments. This appears to be caused
by not being able to conclude Cap <: CapSet^{Cap^}, but both sides
are morally equivalent.

[Linyxus]

A (possible) minimisation:

import language.experimental.captureChecking
import caps.*

trait F[Cap^]
def expect[C^](x: F[C]): x.type = x

def test[C^](x: F[C]): Unit =
  val t1 = expect[CapSet^{C^}](x)