PHPC-1708 introduces return type information for final classes. In interfaces, we can't define return types in PHPC 1.x, as any code implementing the interface contract would no longer be valid. To be able to make this change in 2.0, users will have to update their implementations to already add the return type. Users can add return types to methods that originally don't declare them without causing signature mismatch errors starting with PHP 7.2.

A separate ticket will track adding return types to interface methods.