need `this` for `this` of type `ref @safe Test(string reg_arg)

[rempas via Digitalmars-d-learn]

Ok, so I'm having a struct that has a constructor that takes a 
template parameter. I suppose this question could also be named 
`how to initialize constructors with template parameters` but 
whatever! The funny thing is, I think that I may have already 
found how to do it in the past but I forgot... This time, I'm 
going to add the code in a big module in my project so I'm not 
going to have it as a reference and I won't forget it.

Here is a small sample code:
```d
import std.stdio;

struct Test {
  this(string type = "def")(string reg_arg) {
    writeln("reg_arg: ", reg_arg);
  }
}

void main() {
  auto test = Test.__ctor!("non_def")("Hello");
}
```

And the error I get: `test.d(10): Error: need `this` for `this` 
of type `ref @safe Test(string reg_arg)`