Can’t use UFCS to create InputRange?

[user1234]

On Wednesday, 29 April 2020 at 08:34:53 UTC, Ogi wrote:
> struct R {}
> int front(R r) { return 42; }
> void popFront(R r) {}
> bool empty(R r) { return false; }
>
> void main() {
>     import std.range.primitives : isInputRange;
>     static assert(isInputRange!R);
> }
>
>> Error: static assert:  `isInputRange!(R)` is false
>
> Whats really weird is that if I replace isInputRange with its 
> definition from std.range.primitives, it returns true:
>
> import std;
>
> struct R {}
> int front(R r) { return 42; }
> void popFront(R r) {}
> bool empty(R r) { return false; }
>
> void main() {
>     static assert(is(typeof(R.init) == R)
>             && is(ReturnType!((R r) => r.empty) == bool)
>             && is(typeof((return ref R r) => r.front))
>             && !is(ReturnType!((R r) => r.front) == void)
>             && is(typeof((R r) => r.popFront)));
> }
> This compiles.
>
> What’s going on here?

The static checker doesn't see your free funcs because to do so 
it would have to import the whole module. (is it possible to do 
that ? no idea.)
Also your signature for the primitives are quite unusual (i.e not 
idiomatic). Usually they dont take param. Usually we pass a type 
that contains the member funcs matching to IsIntputRange.