Scoverage: Capability set loses its identity under coverage

[anatoliykmetyuk]

Compiler version

3.10.0-RC1-bin-SNAPSHOT - on latest main as of 6 August 2026, commit 266b5b3afe.

Minimized code

import language.experimental.captureChecking

import java.util.concurrent.atomic.AtomicReferenceFieldUpdater

abstract class Repro[+A](initialTail: (AnyRef | Null)^):
  def makeUpdater: TailUpdater =
    new TailUpdater(
      AtomicReferenceFieldUpdater.newUpdater(
        classOf[Repro[?]],
        classOf[AnyRef],
        "_tail"
      )
    )

final class TailUpdater(u: AtomicReferenceFieldUpdater[Repro[?], AnyRef])

Command

sbt -error -no-colors "scala3-bootstrapped/scalac -color:never -language:experimental.captureChecking -Yexplicit-nulls -coverage-out target/issue-coverage -sourceroot . -d target/issue-out Repro.scala"

Output

-- [E007] Type Mismatch Error: Repro.scala:8:44 -------------------------------
8 |      AtomicReferenceFieldUpdater.newUpdater(
  |                                            ^
  |Found:    (u$1 :
  |  (
  |    java.util.concurrent.atomic.AtomicReferenceFieldUpdater[
  |      Repro[?]{val initialTail: (Object | Null)^{any}}^{}, Object^{}]^{}
  |  )?
  |)
  |Required: java.util.concurrent.atomic.AtomicReferenceFieldUpdater[Repro[?], AnyRef]
  |
  |Note that capability `any²` cannot flow into capture set {any} of value u$1.
  |
  |The error occurred for a synthesized tree:  u$1
  |
  |where:    any  is a root capability in the type of value initialTail
  |          any² is a root capability in the type of value initialTail
1 error found
[error] nonzero exit code returned from runner: 1
[error] (scala3-compiler-bootstrapped / Compile / runMain) nonzero exit code returned from runner: 1

Expectation

Compilation succeeds with coverage instrumentation.

Notes

Is a blocker for the standard library to work with scoverage.