Restrict type of function parameter to a defined list of types?

[Martin B]

Hi everyone,

lets say that required is a setter method:
```
Nullable!string str(Nullable!string setter) {
     return this._str = setter;
}
```

The user should be able to:
```
auto a = new A();
a.str = "abc";
```

As the setters parameter is defined to be of type 
`Nullable!string`, the compiler complains. So the user  need to 
do `a.str = nullable("abc");`

I guess a solution could be to define the setter as:
```
Nullable!string str(T)(T setter) {
     return this._str = setter;
}
```
^stupid question: Is this the correct way to restrict the 
parameter type to be `string` or `Nullable!string` or is there a 
more precise way to restrict?

BTW: here is a more complete simple example of what i am talking 
about: https://run.dlang.io/is/zP4vkb