As stated in various discussion on JDK8, lambda expressions don't allow to retrieve parameter types by reflection.

See:
http://jdk8-dev.openjdk.java.narkive.com/bSPiKxap/jdk-8-lambda-reflection-issues

As section 10.1 of the spec states:

The event types of the event include all superclasses and interfaces of the runtime class of the event object.

In case of a lambda payload, observer resolution will fails when respecting the rule above since parameter type won't be retrieve from the lambda.

For instance the following code

    @Inject
    Event<Supplier<String>> stringSupplierEvent;
...
    Supplier<String> stringSupplier = () -> "Hello world";
    stringSupplierEvent.fire(stringSupplier);

will triggers an observer for Supplier but not an observer for Supplier<String>.