Example code:

```d
module app;
import std;

void main() {
    auto x = new Derived(10, 20);
}

struct Base {
    int a;
}

struct Derived
{
    Base base;
    int b;
    alias this = base;

    @disable this();

    // `a` is not available for use with constructor parameters.
    //
    this( typeof(a) _a, typeof(b) _b ) {
        //
        // inside the body of the constructor, `a` is available!
        //
        a = _a;
        b = _b;

        writeln( a, ", ", b );
    }
}
```

Error:
`Error: undefined identifier a`

Problem:
The compiler can't find the symbol `a` in the constructor parameters, although the symbol was "imported" into the struct scope by using `alias this = base;`.