This was the old GRECLIPSE-1796.

Consider the following code:

package a
import static org.junit.Assert.*
import org.junit.After;
import org.junit.Test

final class A {
	MyBean m = new MyBean()

	class MyBean {
		String doSomething(String a, Integer b) {
		}
	}

	@Test final void test() {
		assertNull(m.doSom|
	}
}

Let "|" be the cursor position and invoke code assist there. Please note the missing closing bracket for assertNull(Object) method invocation, which is a common situation while you're typing some new code.
As you can see, in this case no relevant suggestions are given by code assist (only some obscure - for me at least - get@ and set@ proposals are shown). I think this is related to the editor resilience to typing errors (the code is not valid Groovy code until you close the unpaired bracket), however, as I said, this situation is quite common when you're typing some new code. JDT for Java code works well in this situation, in fact.