what wrong with this alias

[Krzysztof Jajeśnica]

On Sunday, 8 January 2023 at 05:42:46 UTC, Qusatlegadus wrote:
>     auto s = 1234.to!string.map!q{a - '0'}.sum;
> works fine.
>
>
> but if i do an alias
>
>     alias comb = to!string.map!q{a - '0'}
>
>     Error: unknown, please file report on issues.dlang.org
>
> What's wrong with this

## Simple explanation

`to!string` is a function expecting 1 argument, which you're not 
providing in your alias. Convert your alias to a lambda 
expression:

```D
alias comb = x => x.to!string.map!q{a - '0'}
```

## Complicated explanation

`to` is a template defined like this:

```D
// https://github.com/dlang/phobos/blob/master/std/conv.d
template to(T)
{
     T to(A...)(A args)
         if (A.length > 0)
     {
         return toImpl!T(args);
     }
     // some overloads omitted for brevity
}
```

`to` needs at least 2 template arguments - the first one for the 
outer template is passed explicitly (what you did with 
`to!string`), the other ones are inferred from the arguments 
passed to the `to` function. Since you did not pass an argument 
to `to!string`, the inner template doesn't get instantiated.

Basically what happens is you're trying to pass an uninstantiated 
template as argument to `map`. This is quite an exotic situation, 
so probably there isn't a dedicated error message for it or 
you're hitting a bug in the compiler (hence the unknown error 
message).