Description Marc-André Laperle 2015-11-27 02:06:07 EST

Using 4.6-I20151124-1000

I am seeing errors when passing @NonNull to unconstrained parameter. The parameter is generic, so that might have something to do with it.

Here is the snippet:

void example1() {
	@Nullable
	List<String> container = new ArrayList<>();
	@NonNull List<String> list = checkNotNull(container);
}

void example2() {
	@NonNull
	List<String> container= new ArrayList<>();
	@NonNull List<String> list = checkNotNull(container); // Null constraint mismatch: The type '@NonNull List<String>' is not a valid substitute for the type parameter 'C extends Iterable<T>'
}

@NonNull <T, C extends Iterable<T>> C checkNotNull(C container) {
	if (container == null) {
		throw new NullPointerException();
	}
	return container;
}

Here is my understanding of this:
In the checkNotNull method signature, the C in the parameter and T are unconstrained. Only the C returned is constrained. 
In example1, the List is @Nullable and the <String> is unconstrained, which is compatible with the unconstrained C and T in checkNotNull.
In example2, the List is @NonNull and the <String> is unconstrained, which (I thought!) should be compatible with the unconstrained C and T in checkNotNull. But this gives an error.

If I change checkNotNull's parameter to @Nullable C container, example2 works but not example1.