This is a "re-post" of an issue originally opened at Junit: junit-team/junit4#610. As JUnit is going to depreciate and eventually remove ExpectedException I'm re-raising it hear in order to start a discussion whether this change should be made:

Consider the following test:

public class WrongUsageOfExpectedExceptionApi {
    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Test
    public void wrongUsageOfExpectedExceptionApi() throws Exception {
        thrown.expect(RuntimeException.class);

        // thrown.expectMessage() should be used here
        // because an exception cannot start with a string
        thrown.expect(startsWith("some message"));
        throw new RuntimeException("some message");
    }
}
...

When you have something like this in a test it can be quite hard to figure out why the test fails.
At the moment ExpectedException#expect takes a Matcher<?> as its argument. By changing it to take only arguments of the type Matcher<? extends Throwable> the compiler could spot the mistake in the test given above.
I tried the code changes needed for the suggested change in the API with the current master (92a3f73) and all tests pass.
I understand that these changes break the API and non-generic Matchers won't work with the new signature of the method but is there any other reason not to have ExpectedException#expect only take arguments of the type Matcher<? extends Throwable>?
Apart from the exclusion of non-generic Matchers the only downside I see is that now you have to specify the generic type: That is you have to write for example

ExpectedException#expect(CoreMatchers.<Throwable>not(CoreMatchers.<Throwable>instanceOf(type)));

instead of

ExpectedException#expect(not(instanceOf(type)));