@use is deprecated in capture checking, but in the case of capability polymorphism, it is still needed.

[amsen20]

Overview

The following code can be compiled by adding @use before the capability type variable, but in recent versions, there is no way to get this code compiled because @use is deprecated.
There is also a test and a TODO related to this issue, but I don't know whether it is a bug and is going to be fixed or something else.

Compiler version

3.8.0-RC1-bin-20250814-0cf7a18-NIGHTLY

Minimized codes

The base code with capability polymorphism:

//> using scala 3.7.2
import language.experimental.captureChecking

def g[C^] = (xs: List[Object^{C}]) => xs.head

Compile error:

[error] Capture set parameter C leaks into capture scope of method g.
[error] To allow this, the type C should be declared with a @use annotation
[error] def g[C^] = (xs: List[Object^{C}]) => xs.head
[error]                                       ^^^^^^^

Modified version of the code that fixes the issue in 3.7.2:

//> using scala 3.7.2
import language.experimental.captureChecking
import caps.use

def g[@use C^] = (xs: List[Object^{C}]) => xs.head

The base code in 3.8.0-RC1-bin-20250814-0cf7a18-NIGHTLY

//> using scala 3.8.0-RC1-bin-20250814-0cf7a18-NIGHTLY
import language.experimental.captureChecking

def g[C^] = (xs: List[Object^{C}]) => xs.head

Compile error:

[error] Capture set parameter C leaks into capture scope of method g.
[error] def g[C^] = (xs: List[Object^{C}]) => xs.head
[error]                                       ^^^^^^^

The fixed version in 3.8.0-RC1-bin-20250814-0cf7a18-NIGHTLY:

//> using scala 3.8.0-RC1-bin-20250814-0cf7a18-NIGHTLY
import language.experimental.captureChecking
import caps.use

def g[@use C^] = (xs: List[Object^{C}]) => xs.head

Compile error:

[error] @use is redundant here and should no longer be written explicitly.
[error] Capset variables are always implicitly used, unless they are annotated with @caps.preserve.
[error] def g[@use C^] = (xs: List[Object^{C}]) => xs.head
[error]       ^^^^^^^

Expectation

There should be a way to modify the base code and compile it successfully in recent Scala versions.

[Linyxus]

I think this is a bug, and #24328 fixes it. The original example compiles if it is written this way:

import language.experimental.captureChecking
def g[C^](xs: List[Object^{C}]) = xs.head

[amsen20]

Thanks, it fixes the bug. I have a codebase of these examples, so how can I test it on my codebase?