Request for review: proposal for @FunctionalInterface checking

[Joe Darcy]

Hello,

We've had some discussions internally at Oracle about adding a 
FunctionalInterface annotation type to the platform and we'd now like to 
get the expert group's evaluation and feedback on the proposal.

Just as the java.lang.Override annotation type allows compile-time 
checking of programmer intent to override a method, the goal for the 
FunctionalInterface annotation type is to enable analogous compile-time 
checking of whether or not an interface type is functional.  Draft 
specification:

package java.lang;

/**
Indicates that an interface type declaration is intended to be a 
<i>functional interface</i> as defined by the Java Language 
Specification.  Conceptually, a functional interface has exactly one 
abstract method.  Since default methods are not abstract, any default 
methods declared in an interface do not contribute to its abstract 
method count.  If an interface declares a method overriding one of the 
public methods of java.lang.Object, that also does <em>not</em> count 
toward the abstract method count.

Note that instances of functional interfaces can be created with lambda 
expressions, method references, or constructor references.

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

<ul>
<li> The type is an interface type and not an annotation type, enum, or 
class.
<li> The annotated type satisfies the requirements of a functional 
interface.
</ul>

@jls 9.8 Functional Interfaces
@jls 9.4.3 Interface Method Body
@jls 9.6.3.8 FunctionalInterface  [Interfaces in the java.lang package 
get a corresponding JLS section]
@since 1.8
*/
@Documented
@Retention(RUNTIME)
@Target(TYPE)
@interface FunctionalInterface {} // Marker annotation

Annotations on interfaces are *not* inherited, which is the proper 
semantics in this case.  A subinterface of a functional interface can 
add methods and thus not itself be functional.  There are some 
subtleties to the definition of a functional interface, but I thought 
that including those by reference to the JLS was sufficient and putting 
in all the details would be more likely to confuse than clarify.

Please send comments by January 4, 2013; thanks,

-Joe