Improve Captureset Subtyping and Intervals

[bracevac]

From discussions in PR #21875

Compiler version

Latest NIGHTLY

Minimized code

import language.experimental.captureChecking
import caps.*

trait Abstract[X >: Nothing]: // Concrete2 and 3 fail with this lower bound, but work with X >: CapSet^{}
    type C >: X <: CapSet^
    def boom(): Unit^{C^}

class Concrete extends Abstract[CapSet^{}]:
    type C = CapSet^{}
    def boom() = () // ok

class Concrete2 extends Abstract[CapSet^{}]:
    type C = CapSet^{} & CapSet^{}
    def boom() = () // error

class Concrete3 extends Abstract[CapSet^{}]:
    type C = CapSet^{} | CapSet^{}
    def boom() = () // error

Output

-- Error: local/i21868.scala:14:14 ---------------------------------------------
14 |    def boom() = () // error
   |              ^
   |          Illegal capture reference: (caps.CapSet^{}) & (caps.CapSet^{})
-- Error: local/i21868.scala:18:14 ---------------------------------------------
18 |    def boom() = () // error
   |              ^
   |          Illegal capture reference: (caps.CapSet^{}) | (caps.CapSet^{})

Expectation

• I would expect those to work even if the lower bound of X in trait Abstract[X] is set to Nothing.
• Can we make union and intersection on CapSet commute with capture set union and intersection, i.e., CapSet^c1 | CapSet^c2 <:> CapSet^(c1 | c2) and CapSet^c1 & CapSet^c2 <:> CapSet^(c1 & c2)? Probably, that involves improving normalizeCaptures in Setup.
• Can we let the syntax C^ for declaring a type variable automatically desugar to the interval CapSet^{} <: C <: CapSet^?
  • Is this always airtight, or will it cause trouble?

[bracevac]

cc @noti0na1 @Linyxus