Question mark operator proposals

[FeepingCreature]

On Friday, 12 May 2023 at 13:02:25 UTC, Dadoum wrote:
> There were at some point discussions in the forum about error 
> handling and adding a potential question mark operator in D, 
> and I would support it personally since it would allow to force 
> caller of a function to handle some recoverable errors which 
> can happen frequently (especially with `@mustuse`).
>
> ...
> 
> ```d
> struct ServerError {
>     int error;
>     string message;
> }
> 
> alias ServerResponse = SumType!(string, ServerError);
>
> Account account = AuthServer.authenticate(mail, password)?;
> ```

Neat also has this 
(https://neat-lang.github.io/manual.html#sum-type):

```d
struct ServerError {
     int error;
     string message;
}

alias ServerResponse = (string | fail ServerError);

string account = AuthServer.authenticate(mail, password)?;

// More verbose form
string account = AuthServer.authenticate(mail, password)
     .case(ServerError err: return err);
```

`fail` is a keyword of sumtype entries. Any expression of the 
form `x?` will return all entries marked as `fail`, yielding a 
sumtype of the remaining types, or the type if there's only one.

It seems to work pretty well. The interaction with class methods 
is dubious though, because it means that a submethod cannot ever 
return an error that a supermethod didn't account for. (Arguably 
that's not *wrong...*)