How to replicate an Error: scope variable `old` assigned to `ref` variable `ctx` with longer lifetime?

[matheus via Digitalmars-d-learn]

I saw a post 
(https://forum.dlang.org/post/aklmfkzqpsraprjfx...@forum.dlang.org) by Dennis where he gives an example:

Finally, mechanical scope checking has false positives:
```D
void f(ref scope S ctx)
{
   const old = ctx;
   g(ctx);
   ctx = old; // Error: scope variable `old` assigned to `ref` 
variable `ctx` with longer lifetime

}
```

So I'd like to try it out, so I wrote the snippet below:

@safe

import std;

struct S{}

void f(ref scope S ctx) {
    const old = ctx;
    g(ctx);
    ctx = old;
}

void g(ref scope S ctx) {
    S a;
    ctx = a;
}

void main(){
   S i;
   f(i);
}

But no errors. I tried Class too and no errors again.

Could someone please show me how can a replicate?

Thanks,

Matheus.