Opt-in into API implementation

[qwwdfsad]

This issue is for discussion of the proposal to introduce @SubclassOptInRequired annotation in Kotlin to provide a mechanism to require opt-in for API implementation only.

Proposal PR https://github.com/Kotlin/KEEP/pull/319

[nreid260]

This seems like it can be achieved today without adding new primitives:

interface DontSubclassMe {
  @MyExperimentalApi
  fun areYouSureYouWantToSubclassMe()

  fun theRealMethods()
}

[iTob191]

Any reason why OptInRequired was used in the name instead of RequiresOptIn? The latter seems to be more in line with the existing annotation. Suggestions:

• SubclassRequiresOptIn
• RequiresOptInForSubclass
• (RequiresSubclassOptIn)

The first two suggestions share the same term (RequiresOptIn) with the existing annotation which is beneficial for code completion and using the same term also helps to remember the name.