Type dependent annotations?

Hello,

Today you have the following possibilities to specify the element to which an annotation is applicable:

• @Target(ElementType.TYPE)—can be applied to any element of a class
• @Target(ElementType.FIELD)—can be applied to a field or property
• @Target(ElementType.METHOD)—can be applied to a method level annotation
• @Target(ElementType.PARAMETER)—can be applied to the parameters of a method
• @Target(ElementType.CONSTRUCTOR)—can be applied to constructors
• @Target(ElementType.LOCAL_VARIABLE)—can be applied to local variables
• @Target(ElementType.ANNOTATION_TYPE)—indicates that the declared type itself is an annotation type

Wouldn't it be useful to have an extra possibility and restrict the target to elements of a certain type T? For example:

• @Target(T.Field)—can be applied to a field of type T

Couple that to an attribute that specifies if the annotation is required or not on all elements of type T and the compiler can check if an annotation is missing or applied to an element of the wrong type.

I sketched how it could look like in the example below:

@Retention(RetentionPolicy.RUNTIME)
@Target(T.FIELD, required = true)

public @interface TAnnotation {
   public String a();
   public String b();
}


class X {

  @TAnnotation(a="Hello", b="World") // ok
  val f = new T

  @TAnnotation(a="Hello", b="World") // error: annotation only valid for fileds of type T, found U 

  val g = new U

  // error: TAnnotation annotation missing 
  val h = new T 

}

Is there a way to support this without breaking everything? 


Thanks,
Sebastien