Demo project

Steps to reproduce the compiler crash

1. Open the attached Swift Package.
2. CMD+B to build the project.
3. Go to ThreadSafeMockThrowingTypedErrorFunc.swift file for the details what was tried, what compiles and what triggers the crash
4. Compare with MockThrowingTypedErrorFunc.swift, where everything is identical except for this is a final class, not an actor.

Steps to reproduce the runtime crash

1. Open the attached Swift Package with Xcode 26 beta 4
2. CMD+B to build the project
3. CMD+U to run tests
4. You’ll see a crash in ThreadSafeMockThrowingTypedErrorFunc.swift:L39
5. Open CrashingTests.swift for more details

See also the attached stacktraces