Inferring array element type for nested arrays

When the component type of an array creation expression is itself an array creation expression, the element type of the outermost array is not correctly inferred, resulting in type-checking errors.

See file checker-framework/checker/tests/nullness/ArrayCreationSubArray.java, where several array creation expressions do not typecheck: those on the right-hand side of the assignments to o3a, o3b, and o3d. Here is the source code for reference:

public class ArrayCreationSubArray {

    void m() {

        Object o1a = new Integer[] {1, 2, null, 3, 4};
        @Nullable Integer[] o1b = new Integer[] {1, 2, null, 3, 4};

        Object o2a = new Object[] {null};
        Object o2b = new @Nullable Object[] {null};

        Object o3a = new Object[][] {new Object[] {null}};
        Object o3b = new Object[][] {new @Nullable Object[] {null}};
        Object o3c = new @Nullable Object[][] {new @Nullable Object[] {null}};
        Object o3d = new Object[][] {{null}};
    }
}

The type-checker output is:

$ch/bin/javac -processor nullness ArrayCreationSubArray.java 
ArrayCreationSubArray.java:18: error: [array.initializer.type.incompatible] incompatible types in array initializer.
        Object o3a = new Object[][] {new Object[] {null}};
                                     ^
  found   : @FBCBottom @Nullable Object @Initialized @NonNull []
  required: @Initialized @NonNull Object @Initialized @NonNull []
ArrayCreationSubArray.java:19: error: [array.initializer.type.incompatible] incompatible types in array initializer.
        Object o3b = new Object[][] {new @Nullable Object[] {null}};
                                     ^
  found   : @FBCBottom @Nullable Object @Initialized @NonNull []
  required: @Initialized @NonNull Object @Initialized @NonNull []
ArrayCreationSubArray.java:21: error: [array.initializer.type.incompatible] incompatible types in array initializer.
        Object o3d = new Object[][] {{null}};
                                     ^
  found   : @FBCBottom @Nullable Object @Initialized @NonNull []
  required: @Initialized @NonNull Object @Initialized @NonNull []
ArrayCreationSubArray.java:21: error: [array.initializer.type.incompatible] incompatible types in array initializer.
        Object o3d = new Object[][] {{null}};
                                      ^
  found   : null
  required: @Initialized @NonNull Object
4 errors

Start with checker-framework/checker/tests/nullness/ArrayCreationSubArray.java and run the nullness checker on its nested array creation expressions. Trace how component types are inferred for the outer array, then confirm that the assignments to o3a, o3b, and o3d type-check without the reported errors.