[Issue 22860] New: `Error: unknown` with mutually recursive and nested SumType

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22860

          Issue ID: 22860
           Summary: `Error: unknown` with mutually recursive and nested
                    SumType
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Windows
            Status: NEW
          Severity: regression
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: ttanjo at gmail.com

The following code works with dmd 2.098.1 but does not work with dmd 2.099.0
(beta).

```d
import std.sumtype : SumType;

struct None{}

class C1
{   
    SumType!(C1, C2) field;
}

class C2
{
    SumType!(None, SumType!(C1, C2)[]) field;
}

void main() {}
```

dmd 2.099.0 shows the following message:
```console
> rdmd playground.d

Error: unknown, please file report on issues.dlang.org
onlineapp.d(7): Error: template instance `std.sumtype.SumType!(C1, C2)` error
instantiating
```

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