Mutually recursive template expansion

[bauss]

On Friday, 1 October 2021 at 14:26:39 UTC, jfondren wrote:
> On Friday, 1 October 2021 at 14:03:06 UTC, Stephen wrote:
>> This code should work should mutual recursion be supported.
>
> It still wouldn't work, because structs are value types and 
> it's impossible to say how large either struct is:
>
> Error: struct `mutualrec.Ar` no size because of forward 
> reference
>
> With s/struct/class/ it still wouldn't work because this is a 
> mixin problem rather than a problem of template mutual 
> recursion:
>
> ```d
> mixin(q{ class Ar { Br b; } });
> mixin(q{ class Br { Ar b; } });
> ```
>
> mutualrec2.d-mixin-1(1): Error: undefined identifier `Br`, did 
> you mean class `Ar`?
>
> This seems like a surprising limitation of mixin, though, which 
> isn't highlighted by the spec.

Actually it is covered by the spec.

See:

https://dlang.org/spec/expression.html#mixin_expressions

It clearly says:

```
Each AssignExpression in the ArgumentList is evaluated at compile 
time
```

Which means that Br cannot be used in Ar since it cannot be 
evaluated at compile time during the mixin of Ar.