How to get `Type.GenericParameterPosition` for method parameter of class-generic type

It's easy to get the GenericParameterPosition for a parameter when its type is a generic type argument of the method:

public class MyClass {
    public void Foo<T>(T a) { }
}

// ...

Console.WriteLine("pos: " + typeof(MyClass)
    .GetMethod("Foo")
    .GetParameters()
    .Single()
    .ParameterType
    .GenericParameterPosition);

pos: 0

But how can we accomplish this for a parameter who's type is a generic type argument of the class?

public class MyClass<T> {
    public void Foo(T a) { }
}

// ...

Console.WriteLine("pos: " + typeof(MyClass<int>)
    .GetMethod("Foo")
    .GetParameters()
    .Single()
    .ParameterType
    .GenericParameterPosition);

InvalidOperationException: Method may only be called on a Type for which Type.IsGenericParameter is true.