Unexpected assignment.type.incompatible error

[micapolos-google]

This valid code:

interface Array<T extends @Nullable Object> {
  @Nullable T get();
}

interface Cell<C extends Cell<C>> {}

interface Table<C extends Cell<C>> extends Array<C> {}

public static void test(Table<? extends Cell<?>> table) {
  Cell<?> cell = table.get();
}

produces this error:

error: [assignment.type.incompatible] incompatible types in assignment.
    Cell<?> cell = table.get();
                            ^
  found   : capture#01 extends @Nullable Cell<capture#01>
  required: @Nullable Cell<? extends @NonNull Object>

Nullness checker incorrectly assumes that the result of table.get() method is Cell<@Nullable ?>, while in fact it is Cell<?>.

There are currently two ways to overcome this problem:

• add @SuppressWarnings annotation to silence the nullness checker,
• declare local variable as Cell<@Nullable ?> cell, which should not be necessary.

The problem is likely caused by the fact that Table extends Array, where Table has non-null bound, but Array has nullable bounds, so nullness checker confuses the return type of get() method.