Direct extension method use warning

[Thomas Jung]

> (A further problem with use-site extension methods is the lack of reflective
> discovery.  Consumers of classfiles (such as non-Java compilers like JRuby, as
> well as templating mechanism such as JSP EL) cannot easily find these
> extension methods.)

Wouldn't it be useful to annotate a method (e.g. Collection.sort) as
an extension method implementation? Users that invoke the method
directly would get a (deprecation) warning. (The annotation is not
strictly necessary as the extension method declaration makes it
implicitly an extension method implementation.) This warning could
help to move to the "new style".

Thomas