I'm trying to define some structs that contain sumTypes (using
the sumtype library) that refer to each other, but I get a
recursive template instantiation error. It looks like typically
recursion is handled in that library with the This type, but I
don't see how that would apply here. Any advice on how to get
this to work?
```
struct Terminal{
string name;
}
struct Op(string op){
alias ElemType = SumType!(Terminal, Term);
ElemType[] elems;
}
struct Term{
alias ElemType = SumType!(Op!"+", Op!"-", Op!"*", Terminal);
ElemType[] elems;
}
```
Errors with
```
Error: template instance SumType!(Op!"+", Op!"-", Op!"*",
Terminal) recursive template expansion
../../../.dub/packages/sumtype-0.9.4/sumtype/src/sumtype.d(331,14): Error:
template instance std.traits.isCopyable!(Term) error instantiating
source/errorbound/tracked.d(25,20): instantiated from
here: SumType!(Terminal, Term)
source/errorbound/tracked.d(31,29): instantiated from
here: Op!"+"
/usr/local/opt/dmd/include/dlang/dmd/std/meta.d(839,24): Error:
template instance core.internal.traits.allSat!(isCopyable,
Terminal, Term) error instantiating
```
