````
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.

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