https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #1 from Jon Degenhardt <jrdemail2000-dlang@yahoo.com> ---
This is not specific to merge. Here is an example using roundRobin. The reference input range form fails, the value input range works fine. Only the reference input range is shown.

----- case2.d -----
import std.stdio;
import std.range;
import std.algorithm : chunkBy;

class InputRangeClass(R)
{
    R data;
    this(R _data) pure @safe nothrow { data = _data; }
    @property bool empty() pure @safe nothrow { return data.empty; }
    @property auto front() pure @safe nothrow { return data.front; }
    void popFront() pure @safe nothrow { data.popFront(); }
}

auto inputRangeClass(R)(R range) { return new InputRangeClass!R(range); }

void main(string[] args)
{
    auto x1 = [0, 1, 3, 6];
    auto x2 = [0, 2, 4, 6, 7];
    auto x3 = [1, 2, 4, 6, 8, 8, 9];

    auto x =
        roundRobin(x1.inputRangeClass, x2.inputRangeClass, x3.inputRangeClass)
        .chunkBy!((a, b) => a == b);

    writeln(x);
}
-------------------------------

$ dmd case2.d
$ ./case2
core.exception.AssertError@/Users/jondegenhardt/devtools/dmd2-2.083.1/osx/bin/../../src/phobos/std/range/package.d(1779):
Attempting to fetch the front of an empty roundRobin
----------------
??:? _d_assert_msg [0xa7e51a2]
??:? pure nothrow @property @safe int
std.range.roundRobin!(case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass).roundRobin(case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass).Result.front() [0xa7cc5c0]
??:? pure nothrow @safe void
std.algorithm.iteration.ChunkByImpl!(case2.main(immutable(char)[][]).__lambda2,
std.range.roundRobin!(case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass).roundRobin(case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass,
case2.InputRangeClass!(int[]).InputRangeClass).Result).ChunkByImpl.popFront()
[0xa7ccc9f]

... More lines ...

??:? _Dmain [0xa7cc30f]
[[0, 0], [1], [3], [6] $

--

https://issues.dlang.org/show_bug.cgi?id=19532

Jon Degenhardt <jrdemail2000-dlang@yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|chunkBy assert error        |chunkBy assert error
                   |involving merge and         |involving reference input
                   |reference ranges.           |ranges.

--

https://issues.dlang.org/show_bug.cgi?id=19532

Jon Degenhardt <jrdemail2000-dlang@yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|enhancement                 |normal

--

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #2 from Jon Degenhardt <jrdemail2000-dlang@yahoo.com> ---
Another example, this one involving multiwayMerge.

-------- case3.d ---------
import std.stdio;
import std.range;
import std.algorithm : chunkBy, multiwayMerge;

auto inputRangeStruct(R)(R range) { return InputRangeStruct!R(range); }

void main(string[] args)
{
    auto x1 = [0, 1, 3, 6];
    auto x2 = [0, 2, 4, 6, 7];
    auto x3 = [1, 2, 4, 6, 8, 8, 9];
    auto x4 = [1, 3, 5, 6];

    auto y = [x1, x2, x3, x4]
        .multiwayMerge
        .chunkBy!((a, b) => a == b);

    writeln(y);
}
---------------------------

This fails similarly to the other cases.

$ dmd case3.d
jondegenhardt@mac:merge_groupby$ ./case3
core.exception.AssertError@/Users/jondegenhardt/devtools/dmd2-2.083.1/osx/bin/../../src/phobos/std/range/primitives.d(2219):
Attempting to popFront() past the end of an array of int
----------------
??:? _d_assert_msg [0xea781fe]
??:? pure nothrow @nogc @safe void
std.range.primitives.popFront!(int).popFront(ref int[]) [0xea5ee5e]
??:? pure void std.algorithm.setops.MultiwayMerge!("a < b",
int[][]).MultiwayMerge.popFront() [0xea5c623]
??:? pure void
std.algorithm.iteration.ChunkByChunkImpl!(case3.main(immutable(char)[][]).__lambda2,
std.algorithm.setops.MultiwayMerge!("a < b",
int[][]).MultiwayMerge).ChunkByChunkImpl.popFront() [0xea5f494]

... more lines of stack trace ...

??:? _Dmain [0xea5c44d]
[[0, 0], [1, 1, 1], [2, 2], [3, 3], [4, 4], [5], [6, 6, 6, 6], [7$

--

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #3 from Jon Degenhardt <jrdemail2000-dlang@yahoo.com> ---
One more:

------ case4.d ------
import std.stdio;
import std.range;
import std.algorithm : chunkBy, fold, map;
import std.container.binaryheap;

void main(string[] args)
{
    auto x = [3, 1, 0, 6, 0, 2, 4, 7, 6, 8, 8, 4, 1, 2, 9, 5, 3, 1, 6];
    auto y = x
        .heapify!"a > b"
        .chunkBy!((a, b) => a == b)
        .map!(c => c.fold!((a, b) => a + b));

    writeln(y);
}
------------------------

The above will fail with assert errors similar to the others. However, the
following are fine:
   a) Removing the '.map!(c => c.fold...' line.
   b) Adding a '.array' step after 'heapify.

The run:

$ dmd case4.d
$ ./case4
core.exception.AssertError@/Users/jondegenhardt/devtools/dmd2-2.083.1/osx/bin/../../src/phobos/std/algorithm/iteration.d(598):
Attempting to popFront an empty map.
----------------
??:? _d_assert_msg [0xc3c548e]
??:? pure @safe void
std.algorithm.iteration.MapResult!(case4.main(immutable(char)[][]).__lambda3,
std.algorithm.iteration.ChunkByImpl!(case4.main(immutable(char)[][]).__lambda2,
std.container.binaryheap.BinaryHeap!(int[], "a >
b").BinaryHeap).ChunkByImpl).MapResult.popFront() [0xc3ace89]

... more lines ...

??:? _Dmain [0xc3aa1e9]
[0, 3, 4, 6, 8, 5, 18, 7, 16, 9$

--

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #4 from Jon Degenhardt <jrdemail2000-dlang@yahoo.com> ---
A bit more explanation - I encountered problems with chunkBy when writing an merge and aggregation of sorted files. Basically, an external merge sort, except also combining equivalent key entries and aggregating values associated with the keys. This is essentially a merge-chunkby-map/fold operation, except with a custom merge operation.

The examples in this report are cases I found while trying to identify a simpler sample case. These samples still don't show all the behaviors I encountered, but they should be enough to motivate the issues.

--

https://issues.dlang.org/show_bug.cgi?id=19532

Jon Degenhardt <jrdemail2000-dlang@yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|chunkBy assert error        |chunkBy assert error
                   |involving reference input   |involving non-forward input
                   |ranges.                     |ranges.

--

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #5 from Jon Degenhardt <jrdemail2000-dlang@yahoo.com> ---
It turns out that chunkBy has several issues with non-forward input ranges. I am working on a PR to address them.

--

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #6 from Jon Degenhardt <jrdemail2000-dlang@yahoo.com> ---
PR 6841 (https://github.com/dlang/phobos/pull/6841)

--

https://issues.dlang.org/show_bug.cgi?id=19532

--- Comment #7 from github-bugzilla@puremagic.com ---
Commits pushed to master at https://github.com/dlang/phobos

https://github.com/dlang/phobos/commit/b17984059e3dd0a0a4afc696b7f5696bf3c54826 Fix Issue 19532 - chunkBy errors involving non-forward input ranges

https://github.com/dlang/phobos/commit/98d74d5c16923ca0460594b8e230dea885cf591d Merge pull request #6841 from jondegenhardt/fix-19532-chunkBy-input-range

Fix Issue 19532 - chunkBy errors involving non-forward input ranges merged-on-behalf-of: Nicholas Wilson <thewilsonator@users.noreply.github.com>

--