Unclear error message

[SealabJaster]

Please see the code at https://run.dlang.io/is/Yjidek

As I understand the error is caused by trying to provide a 
delegate when there's no context to provide. Not complaining 
about that.

However what I am complaining about is about the error message: 
`onlineapp.d(31): Error: delegate onlineapp.S.__lambda2 cannot be 
struct members`

I'm not sure if it's just me, but that error message makes 
absolutely no sense to me. Should that message be improved?