Annotates methods of a Module to add items to a Multibinder. The method's return type and binding annotation determines what Optional this will contribute to. For example,

@ProvidesIntoOptional(DEFAULT)
@Named("url")
String provideFooUrl(FooManager fm) { return fm.getUrl(); }

@ProvidesIntoOptional(ACTUAL)
@Named("url")
String provideBarUrl(BarManager bm) { return bm.getUrl(); }

will set the default value of @Named("url") Optional<String> to foo's URL, and then override it to bar's URL.