The following code compiles and runs just fine with Eclipse 2022-12 (4.26.0).
public static void main(String[] args) {
List<List<?>> list = Arrays.asList(Arrays.asList("test"));
System.out.println(test(list));
}
public static <T> T test(List<? extends List<T>> list) {
return list.get(0).get(0);
}
However, when compiling with OpenJDK javac 19.0.2 I get the following error.
error: method test in class Test cannot be applied to given types;
System.out.println(test(list));
^
required: List<? extends List<T>>
found: List<List<?>>
reason: cannot infer type-variable(s) T
(argument mismatch; List<List<?>> cannot be converted to List<? extends List<T>>)
where T is a type-variable:
T extends Object declared in method <T>test(List<? extends List<T>>)
Which compiler is correct in this case, and why? Is there a JLS that specifies that this assignment is (not) allowed? I have also read this answer. However, this doesn't help me to figure out if the assignment is allowed or not because the case of a generic type parameter T is not treated there.
Here is an example where this is relevant.
ExecutorService executorService = /* ... */;
List<Callable<?>> callables = /* ... */;
executorService.invokeAll(callables);
This code compiles with Eclipse but not with OpenJDK javac.
