SumType alias can't be deduced?

[Paul Backus]

On Monday, 21 February 2022 at 18:43:18 UTC, Emmanuelle wrote:
> If you run this, the compiler should emit this error:
>
> ```d
> onlineapp.d(14): Error: template `onlineapp.foobar` cannot 
> deduce function from argument types `!()(SumType!(int, Unit))`
> onlineapp.d(8):        Candidate is: `foobar(T)(Option!T 
> option)`
> ```
>
> If you change `foobar`’s parameter’s type from `Option!T` to 
> `SumType!(T, Unit)`, it  compiles without error.

This is a long-standing limitation of the D compiler's template 
argument deduction: it cannot "see through" `alias` templates to 
deduce the underlying type.

The enhancement request to make this work was submitted in 2008: 
https://issues.dlang.org/show_bug.cgi?id=1807

Unfortunately, getting the compiler to handle this correctly is 
much harder than it looks, and nobody has managed to do it yet, 
though several have tried.

As a workaround, you can define your `Optional` type using a 
struct instead of an `alias`:

```d
import std.sumtype;

struct Unit {}
struct Option(T) {
     SumType!(T, Unit) data;
     this(U, this This)(U value)
     {
         data = value;
     }
     alias data this;
}

void foobar(T)(Option!T option) {}

void main() {
     foobar(Option!int(123));
}
```