Implicit conversion by return

[Jonathan M Davis]

On Wednesday, August 8, 2018 2:15:16 AM MDT Hakan Aras via Digitalmars-d-
learn wrote:
> Given this:
>
> struct Num
> {
>      this(int a) {}
> }
>
> Is there any reason why this works:
>
> Num n = 5;
>
> but this doesnt:
>
> Num funk()
> {
>      return 5;
> }
>
>
> I understand that I can construct it explicitely, but that gets
> annoying quickly, especially with templates.

Num n = 5;

doesn't actually do an implict conversion. It's the same as doing

Num n = Num(5);

So, I guess that you could call it implicit construction, but regardless,
it's just a different syntax for calling the constructor. The only way to
create an implicit conversion with a user-defined type in D is to use alias
this, and that only provides a way to implicitly convert _from_ a
user-defined type, not to a type. So, having

Num funk()
{
    return 5;
}

work is impossible in D, just like having something like

auto foo(Num n)
{
    ...
}

foo(5);

work is impossible. If you want to return an int and have it converted to a
Num, then you're going to need to explicitly construct a Num from the int.

- Jonathan M Davis