[Issue 3872] std.algorithm.filter could become bidirectional if its input range is bidir

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=3872

FeepingCreature <default_357-line at yahoo.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |default_357-line at yahoo.de
         Resolution|FIXED                       |---

--- Comment #2 from FeepingCreature <default_357-line at yahoo.de> ---
Got unfixed at some point:

```
import std.algorithm : filter;
import std.range : only;
void main()
{
    int[] a = [2, 3];
    assert(only(2, 3).back == 3);
    assert(a.filter!"a == 2".back == 2);
}
```

=> 

onlineapp.d(7): Error: no property `back` for `filter(a)` of type
`std.algorithm.iteration.FilterResult!(unaryFun, int[])`

Apparently there's `filterBidirectional`. I am violently opposed to that
function and all it represents. `filter` should support bidirectional iteration
by default, with maybe `filterUnidirectional` or `forceForwardRange` if you
want to *disable* it. Since when do we sacrifice functionality for speed by
default?

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