Context function is eagerly applied inside a capture set

[reftrans]

Compiler version

Scala 3.2.2 and latest snapshot (250757732398b92cb6fc8f02e50526573f1ba2b9).

Minimized code

import language.experimental.captureChecking
def fn(ap: String ?=> Int)(using str: String): {ap} Int = ap(using str)

Output

Compile error:

4 |def fn(ap: String ?=> Int)(using str: String): {ap} Int = ap(using str)
  |                                                ^^
  |                                                ap.apply(str): Int is not a legal element of a capture set
1 error found

It compiles as expected if ap is a normal (non-context) function.

Expectation

Successful compilation, like with a normal function.

[prolativ]

For reference: other issues, where too eager application of context functions caused some problems:
#16540
#16506
#16488
#16035
#15995

[reftrans]

This problem is actually more simple than I thought. This demonstrates the problem:

import language.experimental.captureChecking
def fn(ap: String ?=> Int): Int^{ap} = ap(using "")

It's not possible to refer to the context function, ap without its parameter getting applied. Even writing it as a @retains annotation and trying to force it stay as an unapplied context function does not work.

[Gedochao]

Note: if I'm not mistaken, the minimized code from the original post has a typo, I believe it should be:

import language.experimental.captureChecking
def fn(ap: String ?=> Int)(using str: String): Int^{ap} = ap(using str)

This produced the initially reported error on the current main:

[info] running (fork) dotty.tools.dotc.Main -classpath /Users/pchabelski/Library/Caches/Coursier/v1/https/repo1.maven.org/maven2/org/scala-lang/scala-library/2.13.12/scala-library-2.13.12.jar:/Users/pchabelski/IdeaProjects/dotty2/library/../out/bootstrap/scala3-library-bootstrapped/scala-3.4.0-RC1-bin-SNAPSHOT-nonbootstrapped/scala3-library_3-3.4.0-RC1-bin-SNAPSHOT.jar -d . smth.scala
-- Error: smth.scala:2:52 ------------------------------------------------------
2 |def fn(ap: String ?=> Int)(using str: String): Int^{ap} = ap(using str)
  |                                                    ^^
  |                ap.apply(str): Int is not a legal element of a capture set
1 error found
[error] Nonzero exit code returned from runner: 1