What does @retroactive mean on a Swift protocol conformance?

What does @retroactive mean on a Swift protocol conformance?

1. The module acknowledges that it owns neither the type nor protocol and accepts the conformance-conflict risk.
2. The conformance is visible only inside the declaring source file.
3. The runtime dynamically removes the conformance if an upstream module adds one.
4. The compiler gives this conformance priority over all other implementations.