What is "FilterResult" type?

[Bahman Movaqar via Digitalmars-d-learn]

On Tuesday, 8 September 2015 at 18:45:33 UTC, Jonathan M Davis 
wrote:
> If you're returning a range, you should be returning auto.

@Jonathan, @cym13 and @Meta
It's reasonable to use `auto`.  However there are times when you 
need to pass the `auto` value to another function and the 
receiving function needs to know the type of its input arguments.

In other news:
Consider the following piece of code.  *Please note that I am 
aware that the simple problem introduced can be solved in several 
ways which are more efficient.*

     immutable struct FooBar {
       int x;
       int y;
     }
     immutable(FooBar)[] foobars = [
       FooBar(60, 100),
       FooBar(8, 20),
       FooBar(9, 15)
     ];
     int[] nums = [20, 40, 55];

     //////////////////////////////////////////
     // find FooBars which:
     //   - x * y is greater than any `num`
     //   - x and y are smaller than any `num`
     //////////////////////////////////////////
     auto result = reduce!(
       (acc, num) => acc.filter!(
         fb => (fb.x < num && fb.y < num) && (fb.x * fb.y > num)
       )
     )(foobars, nums);
     //
     assert(result[0].x == 9 && result[0].y == 15);

This fails to compile with the following message:

     Error: static assert  "Incompatible function/seed/element: 
test.__unittestL40_4.__lambda1/immutable(FooBar)[]/int"
     /usr/include/dmd/phobos/std/algorithm/iteration.d(2518):      
   instantiated from here: reduceImpl!(false, int[], 
immutable(FooBar)[])
     /usr/include/dmd/phobos/std/algorithm/iteration.d(2502):      
   instantiated from here: reducePreImpl!(int[], 
immutable(FooBar)[]) test.d(56):        instantiated from here: 
reduce!(immutable(FooBar)[], int[])

The only way to make it work is `.array.idup` the output of 
`filter`.  For example:

     auto result = reduce!(
       (acc, num) => acc.filter!(
         fb => (fb.x < num && fb.y < num) && (fb.x * fb.y > num)
       ).array.idup
     )(foobars, nums);

Honestly, I cannot comprehend anything out of this; why I had to 
realise the lazy value and `idup` it for it to become usable by 
`reduce`?
Does this mean that I am simply missing something obvious?