[Issue 20458] CRTP + pass alias to virtual member to mixin = runtime crash

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20458

--- Comment #4 from Basile-z <b2.temp at gmx.com> ---
Finally the generic reproduction

```
class Base
{
    alias a = Derived.test;
    pragma(msg, typeof(a)); // triggers the vtbl corruption
}

class Derived : Base
{
    void test(){}
}

void main()
{
    (new Derived).test();
}   
```

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