[Issue 19823] New: std.range.dropOne doesn't drop the element when called after std.algorithm.iteration.filter

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19823

          Issue ID: 19823
           Summary: std.range.dropOne doesn't drop the element when called
                    after std.algorithm.iteration.filter
           Product: D
           Version: D2
          Hardware: x86
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: jonas.alves at gmail.com

It seems that in some cases dropOne doesn't work when called after
std.algorithm.iteration.filter. In the example I'm pasting below it seems that
it only works when the 1st position in the array is not filtered out.
In production we had more complex cases where it would do the correct thing
when compiled with dmd, but it would not work when compiled with ldc2.
Anyone knows why? I didn't look at the source code, but it looks like buggy
code generation.

import std.range: dropOne;
import std.stdio: writefln;
import std.algorithm.iteration: filter;
import std.array: array;

void main() {
    auto a = [1,2,3,4];
    writefln("%s", a.filter!(a => a != 1).dropOne); // [2, 3, 4] WRONG
    writefln("%s", a.filter!(a => a != 2).dropOne); // [3, 4]
    writefln("%s", a.filter!(a => a != 3).dropOne); // [2, 4]
    writefln("%s", a.filter!(a => a != 4).dropOne); // [2, 3]
    writefln("%s", a.filter!(a => a == 1).dropOne); // []
    writefln("%s", a.filter!(a => a == 2).dropOne); // [2] WRONG
    writefln("%s", a.filter!(a => a == 3).dropOne); // [3] WRONG
    writefln("%s", a.filter!(a => a == 4).dropOne); // [4] WRONG
    writefln("%s", a.filter!(a => a != 1).array.dropOne); // [3, 4]
}

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