In practice, it shows that implementing a non-feature interface or extending a non-feature class is a common mistake, that's quite confusing. Make this an error, so the processor can point the problem out more effectively.
Now, "importing" foreign interfaces into a feature may be a useful strategy to support integration with foreign code. This should definitely be supported, but make this an explicit act by adding a @Feature.Import annotation, which suppresses the error.
In practice, it shows that implementing a non-feature interface or extending a non-feature class is a common mistake, that's quite confusing. Make this an error, so the processor can point the problem out more effectively.
Now, "importing" foreign interfaces into a feature may be a useful strategy to support integration with foreign code. This should definitely be supported, but make this an explicit act by adding a
@Feature.Import
annotation, which suppresses the error.