This is somewhat the old GRECLIPSE-1426.

Consider the following Java interface:

package test;

public interface I {
	void myMethod(String a, int b);
}

And the following Groovy class:

package test

class B { 
	void test() { 
		I myvar = new I(){ 
			 |
		} 
	} 
}

Invoke code assist at "|": I would expect to see the following proposal:

myMethod(String a, int b): void - Override method in 'I'

(indeed, it should be "implement missing method", rather than "override", but it's not that important)

However, no proposal at all is shown.