Signifies that changes in this experimental API can cause compilation errors or warnings in the client code.

Non-signature usages (inside a function body, variable initializer, default argument value, etc.) of compilation-affecting experimental API are allowed either if the containing declaration is annotated with the experimental annotation marker and thus propagates the experimental aspect to its clients, or if there's a UseExperimental annotation entry with the corresponding annotation marker somewhere above that usage in the parse tree. Signature usages of compilation-affecting experimental API always require propagation (as long as the experimental API is declared in another module).