When do Java generics require extends T instead of T and is there any downside of switching

Question

Given the following example (using JUnit with Hamcrest matchers):

Map<String, Class<? extends Serializable>> expected = null;
Map<String, Class<java.util.Date>> result = null;
assertThat(result, is(expected));  

This does not compile with the JUnit assertThat method signature of:

public static <T> void assertThat(T actual, Matcher<T> matcher)

The compiler error message is:

Error:Error:line (102)cannot find symbol method
assertThat(java.util.Map<java.lang.String,java.lang.Class<java.util.Date>>,
org.hamcrest.Matcher<java.util.Map<java.lang.String,java.lang.Class
    <? extends java.io.Serializable>>>)

However, if I change the assertThat method signature to:

public static <T> void assertThat(T result, Matcher<? extends T> matcher)

Then the compilation works.

So three questions:

1. Why exactly doesn't the current version compile? Although I vaguely understand the covariance issues here, I certainly couldn't explain it if I had to.
2. Is there any downside in changing the assertThat method to Matcher<? extends T>? Are there other cases that would break if you did that?
3. Is there any point to the genericizing of the assertThat method in JUnit? The Matcher class doesn't seem to require it, since JUnit calls the matches method, which is not typed with any generic, and just looks like an attempt to force a type safety which doesn't do anything, as the Matcher will just not in fact match, and the test will fail regardless. No unsafe operations involved (or so it seems).

For reference, here is the JUnit implementation of assertThat:

public static <T> void assertThat(T actual, Matcher<T> matcher) {
    assertThat("", actual, matcher);
}

public static <T> void assertThat(String reason, T actual, Matcher<T> matcher) {
    if (!matcher.matches(actual)) {
        Description description = new StringDescription();
        description.appendText(reason);
        description.appendText("\nExpected: ");
        matcher.describeTo(description);
        description
            .appendText("\n     got: ")
            .appendValue(actual)
            .appendText("\n");

        throw new java.lang.AssertionError(description.toString());
    }
}