Error: this for needs to be type S not type MapResult!...

[Peter Neubauer]

Please explain why this error happens in the following code:
import std.algorithm;

struct S
{
  void foo ()
  {
    int f1 (int a) { return conv(a); }
    int delegate (int) f2 = &conv;

    int[] x = [1, 2, 3];
    x.map!conv;    // ERROR
    x.map!f1;      // fine
    x.map!f2;      // also fine
  }

  int conv (int a)
  {
    return a+1;
  }
}

--- compile output:

/usr/include/d/std/algorithm.d(404): Error: this for conv needs to be 
type S not type MapResult!(conv, int[])
/usr/include/d/std/algorithm.d(438): Error: this for conv needs to be 
type S not type MapResult!(conv, int[])
/usr/include/d/std/algorithm.d(450): Error: this for conv needs to be 
type S not type MapResult!(conv, int[])
/usr/include/d/std/algorithm.d(390): Error: template instance 
std.algorithm.MapResult!(conv, int[]) error instantiating
/home/peter/proggen/goliza.reduced/gtp.d(12):        instantiated from 
here: map!(int[])
/home/peter/proggen/goliza.reduced/gtp.d(12): Error: template instance 
std.algorithm.map!(conv).map!(int[]) error instantiating

---

Thanks,
-Peter