Confusing error when assigning result of ref readonly-returning method to a ref local

[svick]

Version Used: 2.8.2; master from 14 July 2018 (c4189de)

Steps to Reproduce:

Try to assign the result of a ref readonly-returning method to a non-readonly ref local:

class C
{
    void M()
    {
        ref int x = ref Bar();
    }
    
    ref readonly int Bar() => throw null;
}

Actual Behavior:

error CS8329: Cannot use method 'C.Bar()' as a ref or out value because it is a readonly variable

I think this is not a good error message because:

1. It's not correct: method is not a variable.
2. It does not explain well what the error is and how to fix it.