NullableOnContainingClass
Type-use nullability annotations should annotate the inner class, not the outer class (e.g., write `A.@Nullable B` instead of `@Nullable A.B`).

The problem

The correct syntax to apply a TYPE_USE annotation to an inner class is A.@Nullable B.

For a TYPE_USE @Nullable annotation, @Nullable A.B is legal Java if B is a non-static inner class:

class A {
  @Target(TYPE_USE)
  @interface Nullable {}

  class B {}
  static class C {}

  void test(A.@Nullable B x) {} // B is annotated ('A' is the enclosing instance type)
  void test(A.@Nullable C x) {} // C is annotated ('A' is a 'scoping construct' here)
}

  void test(@Nullable A.B x) {} // compiles, but likely incorrect: annotates the enclosing instance type 'A', which can never be null
  void test(@Nullable A.C x) {} // compile error: 'A' cannot be annotated

However, for @Nullable (and @NonNull, and friends), annotating the outer class is meaningless. The reference to the outer class (A.this) can never be null, so any nullability annotations are redundant.

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("NullableOnContainingClass") to the enclosing element.