Adjustments to the capability trilogy

[Linyxus]

Fixes #23421.

This PR includes the following two changes:

1. In capToFresh, turn caps in invariant occurrences also to Freshes. Previously, in the follow code:

val xs: ArrayBuffer[() ->{cap} Unit] = ...

The cap is kept intact since it is invariant. Now, it gets converted into a Fresh owned by xs.
2. In toResultInResults, convert fresh caps of def-method with only type parameters into result caps, just like parameterless defs. Specifically, for the following code:

def empty[T]: ArrayBuffer[T]^ = ...

The ^ will be turned into a result cap, just like what happens for a parameterless def:

def mkRef: Ref^ = ...

The ^ is a result cap that gets instantiated to a new fresh at each application of mkRef. Doing the same thing for empty is the arguably more desirable behavior.

[Linyxus]

To be merged after #23427.

[Copilot]

Pull Request Overview

This PR improves the conversion of capabilities by turning invariant cap occurrences into Freshes and converting fresh caps of def methods with only type parameters into result caps. Key changes include:

• Converting invariants to Fresh ownership in capToFresh.
• Adapting toResultInResults to also cover def methods with type parameters.
• Refactoring internal utilities to use deep boxing (boxDeeply) and adjusting variance handling in capability mapping.

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/pos-custom-args/captures/singleton-subtyping.scala	Updates to capability conversion and singleton subtyping tests.
tests/neg-custom-args/captures/i16725.scala	Adjusted inline error comments for wrapped IO usage.
compiler/src/dotty/tools/dotc/cc/Setup.scala	Replaced usages of box with boxDeeply and refined defaultApply behavior.
compiler/src/dotty/tools/dotc/cc/Capability.scala	Refined the variance check in capability mapping from <= to <.
...	Other files show updates to test expectations and diagnostic messages.

Comments suppressed due to low confidence (3)

tests/neg-custom-args/captures/i16725.scala:10

• The inline error comment on this line is now misleading since the expected behavior has changed; please update or remove the comment to reflect the current expected behavior.

    wrapper.value: io =>

compiler/src/dotty/tools/dotc/cc/Capability.scala:696

• The change from 'if variance <= 0' to 'if variance < 0' refines the handling of invariant positions; please verify that this update aligns with the intended capability variance semantics.

      if variance < 0 then t

compiler/src/dotty/tools/dotc/cc/Setup.scala:228

• Replacing 'box' with 'boxDeeply' improves the safety of nested capture conversions; double-check that this change preserves the intended behavior across all application sites.

        tp.derivedAppliedType(tycon, args.mapConserve(_.boxDeeply))

[odersky]

Otherwise LGTM