isInputRange not satisfied even if all individual conditions are satisfied

[Johannes Loher]

Am 26.06.20 um 15:35 schrieb ag0aep6g:
> `isInputRange!R` fails because it has no knowledge of your free `empty`
> function. Without `empty`, `R` is obviously not a range.

Ah, OK, that makes sense. It's kind of sad though because it really
limits the extensibility of existing types with UFCS. Do you know if
there is a way around this?