Description Srikanth Sankaran 2013-02-09 23:16:42 EST

0.6.1 - 9.8, Discussion & motivation box, second bullet says:

// --------8< -----------------


Under the current definition, a functional interface may be parameterized in a way that produces distinct abstract methods—that is, multiple methods that cannot be legally overridden with a single declaration. For example: 
interface I { Object m(); }
interface J<S> { S m(); }
interface K<T> { T m(); }
interface Functional<S,T> extends I, J<S>, K<T> {}
Interface Functional is functional—I.m is return-type-substitutable for the other two—but Functional<String,Integer> clearly cannot be implemented with a single method. This situation is acceptable, however: when functional interfaces are implicitly implemented, the overriding effectively occurs generically; instantiation of type parameters only occurs at the use site. In other words, if a lambda expression represents a Functional<String,Integer>, its treatment is similar to that of the following: 
class FunctionalImpl<S,T> implements Functional<S,T> { ... }
Functional<String,Integer> f = new FunctionalImpl<String,Integer>();

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I think this part needs a careful relook - Both javac 8b74 and eclipse refuse to compile this snippet *precisely* for the reason that certain parameterization could run into unrelated  return types.
Ergo, `Functional' is not even a legal interface - let alone a functional interface.