internal class A
{
}

internal class B
{
    public class C : A { }
}

Note that effective accessiblity of C is 'internal', so it is OK that it is derived from internal class.
Apply refactoring 'move to outer scope' to nested class C.

internal class A
{
}

internal class B
{
}

public class C : A { } /* error CS0060: Inconsistent accessibility: base class 'A' is less accessible than class 'C' */

Preferred result:

internal class A
{
}

internal class B
{
}

internal class C : A { } /* accessibility OK */

Issue was closed