Why does @FunctionalInterface have a RUNTIME retention?

Question

Said in Javadoc:

If a type is annotated with this annotation type, compilers are required to generate an error message unless ...

Answer 1

"Source" would not be enough, since if for example you create an API and provide your class as a pre-compiled jar, the information would not be available for the compiler anymore.

I believe "class" would also not be enough if you want to support those kind of compilers that "compile" against a class at runtime, like scripting engines that use reflection to find out about those annotations and should show a warning, too.