Java 8 gave us many fun ways to use functional interfaces and with them a new annotation: [@FunctionalInterface](http://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html). Its job is to tell the compiler to yell at us if we fail to stick to the rules of a functional interface (only one abstract method that needs overriding please).

There are [43 interfaces in the java.util.function package](https://stackoverflow.com/questions/27743315/a-summary-of-the-parameters-and-return-type-of-functional-interfaces-in-the-pack/28162720#28162720) with this annotation. A search of jdk.1.8.0/src for `@FunctionalInterface` only turns up 57 hits. Why are the other interfaces (such as AutoCloseable) that could have added `@FunctionalInterface` still missing it?

There is a bit of a vague hint in the [annotations documentation](http://docs.oracle.com/javase/8/docs/api/java/lang/FunctionalInterface.html):

"An informative annotation type used to indicate that an interface type declaration is **intended** to be a functional interface" 

Is there any good reason NOT to **intend** that an interface I've designed (that may simply happen to be a functional interface) not be used as one? Is leaving it off an indication of anything besides not realizing it could have been added? 

Isn't adding abstract methods to any published interface going to screw anyone implementing it, functional or not? I feel cynical assuming they just didn't bother to hunt them all down but what other explanation is there?

**Update**: After looking over ["Should 'Comparable' be a 'Functional interface'?"](https://stackoverflow.com/questions/25222575/should-comparablet-be-a-functional-interface) I find I still have nagging questions. When a Single Method Interface and a Functional Interface are structurally identical what's left to be different? Is the difference simply the names? Comparable and Comparator are close enough to the same semantically. Turns out they are different structurally though so still not the best example... 

Is there a case when an SMI is structurally fine to use as a Functional Interface but still discouraged over the semantic meaning of the name of the interface and the method? Or perhaps the contract implied by the Javadocs?