What's wrong in this templatized operator overload ?

[Mike Parker via Digitalmars-d-learn]

On Thursday, 22 October 2015 at 03:19:49 UTC, MobPassenger wrote:
> code:
> ---
> struct Foo
> {
>     bool opIn_r(T)(T t){return false;}
> }
>

This needs to be marked with const:

struct Foo
{
     bool opIn_r(T)(T t) const {return false;}
}