Error CS0122 when attempting to implement a nested `private protected` interface in descendant classes

**Version Used**: Visual Studio 16.5.4

```
public class X: X.XI, X.XII
{
private interface XI
{
}

private protected interface XII
{
}
}

public class XX: X, X.XII
{
public void Test()
{
XII z = this;
}
}
```

**Expected Behavior**:

The class type `XX` would be able to implement the `X.XII` interface.

**Actual Behavior**:

The error "CS0122: 'X.XII' is inaccessible due to its protection level" is generated.