Unable to convert a MapResult to an InputRange.

[Jake Pittis via Digitalmars-d-learn]

````
import std.stdio;
import std.array;
import std.algorithm;
import std.range;

InputRange!string keys(int[string] foo) {
   return foo.byPair.map!(p => p[0]);
}

void main() {
   int[string] foo;
   foo["lol"] = 1;
   foo["wat?"] = 2;
   keys(foo).each!(p => writeln(p));
}
````

The above code produces an `cannot implicitly convert expression 
(map(byPair(foo))) of type MapResult!(__lambda2, 
MapResult!(__lambda2, Result)) to 
std.range.interfaces.InputRange!string` error.

If I replace `InputRange!string` with `auto`, it compiles.

I'm intending to use InputRange!string as a return type in an 
interface so I can't use auto.

````
interface bar {
   InputRange!string keys();
}
````

I don't want to convert it to an array because of the allocation. 
Isn't the whole point of ranges to pass around a lazy interface 
without having to allocate any extra memory? Why can't I cast the 
output of map to an InputRange or something similar?

Thanks for the help. :) Phobos documentation is lovely but I've 
been finding errors similar to this to be quite frustrating.