isInputRange not satisfied even if all individual conditions are satisfied

[ag0aep6g]

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

To be clear: It's the same with `front` and `popFront`. You can't 
implement any range primitives as free functions.

It only works for arrays because those implementations are part of 
std.range.