Invariance of MockitoHamcrest#argThat

[foaw]

The current signature of MockitoHamcrest#argThat makes it impossible to pass in arguments that are not directly of type T.

mockito/src/main/java/org/mockito/hamcrest/MockitoHamcrest.java

Lines 77 to 81 in f5449f9

	@SuppressWarnings("unchecked")
	public static <T> T argThat(Matcher<T> matcher) {
	reportMatcher(matcher);
	return (T) defaultValue(genericTypeOfMatcher(matcher.getClass()));
	}

In particular, this means, for example, that one cannot use Matcher<Iterable<T>> in places where Matcher<Collection<T>> is expected, or even the most primitive Matchers.notNullValue(). The only other remedy is to declare a matcher per type, or "monomorphise" it.

This was already brought up some years ago, but then was closed, as MockitoHamcrest was planned for deprecation anyway. After some discussion in #1819, the deprecation commit was reverted in ad2f352. However, the issue remained unaddressed, and so the problem is still there.

[LeMikaelF]

Here's a possible workaround:

private <T> Matcher<T> adapt(Matcher<? super T> matcher) {
    return new CombinableMatcher<>(matcher);
}

Usage:

class MyClass {
    void doSomething(List<String> list) { }
}
MyClass mock = mock(MyClass.class);
mock.doSomething(List.of("first", "second"));
verify(mock).doSomething(argThat(adapt(hasItems("first"))));

[LeMikaelF]

@foaw This is a problem that I also experience on a regular basis, and I find myself using the above helper method in these situations. However the fix would be really simple, would likely not break any code, and would make using hamcrest matchers a lot easier. I for one would like to see this merged in Mockito (but I'm not a maintainer). Maybe open a PR with this?