Mutually recursive template expansion

[jfondren]

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.