detekt: UnnecessaryAbstractClass false positive

Expected Behavior

Don’t report following code:

abstract class Fake : Base() // UnnecessaryAbstractClass
abstract class Base {
	open val order: Int get() = 0
	abstract fun String.foo()
}

Observed Behavior

“An abstract class without a concrete member can be refactored to an interface.”

No, it can’t:

• Fake interface couldn’t extend a class.
• it has concrete members inherited from Base
• I get a detection even if I add an actual class using the “abstractness”:

class Foo : Fake() {
	override fun String.foo() = TODO("not implemented")
}

Note: String. might be a red herring, you can ignore that part, just wanted to report what I had. More minimal repro might exist.

Steps to Reproduce

Just copy above code into a test or code checked by Detekt 1.20.0.

Context

Real context is creating fakes for a test:

The abstract method is never invoked in the test, so there’s no need to implement it, hence using abstract class. The class must be unique because it’s ::class object is used.

Your Environment

• Version of detekt used: 1.20.0
• Version of Gradle used (if applicable): N/A
• Gradle scan link (add --scan option when running the gradle task): N/A
• Operating System and version: all
• Link to your project (if it’s a public repository): https://github.com/TWiStErRob/net.twisterrob.cinema/pull/114/commits/0a43c8e914bcd46fd9db4a80baeba0470a7e07df in https://github.com/TWiStErRob/net.twisterrob.cinema/pull/114