Wrong CaptureSet comparison for singleton types

[noti0na1]

Minimized code

import language.experimental.captureChecking

class A:
  val m: A^ = this

def test(a: A^, b: A^) =
  type at = a.type
  val c1: at^{b} = a
  val c2: c1.type = a

Output

-- [E007] Type Mismatch Error: Stest.scala:207:19 ------------------------------
207 |  val c1: at^{b} = a
    |                   ^
    |                   Found:    (a : A^)
    |                   Required: at^{b}
    |---------------------------------------------------------------------------
    | Explanation (enabled by `-explain`)
    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    |
    | Tree: a
    | I tried to show that
    |   (a : A^)
    | conforms to
    |   at^{b}
    | but none of the attempts shown below succeeded:
    |
    |   ==> (a : A^)  <:  at^{b}
    |     ==> A^{a}  <:  at^{b}
    |       ==> A  <:  at^{b}
    |         ==> A  <:  at
    |           ==> A  <:  (a : A^)  = false
    |
    | The tests were made under the empty constraint
     ---------------------------------------------------------------------------
1 error found

Expectation

Although we cannot attach a capture set to a singleton type directly, this check can be easily bypassed by declaring a type alias or substitution.

This example should compile.