[Issue 19220] New: multiSort and SortedRange.groupBy do not play together

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19220

          Issue ID: 19220
           Summary: multiSort and SortedRange.groupBy do not play together
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: mrjnewt at gmail.com

std.algorithm.multiSort returns a SortedRange which in turn has a groupBy
method.  Usage of this method does not actually compile when sorting by more
than one predicate.  Example repro (online here:
https://run.dlang.io/is/mQGAvD):

import std.stdio;
import std.algorithm;
void main()
{
        auto records = [
        Record(10, 10),
        Record(12, 1),
        Record(30, 12),
        Record(10, 10),
    ];

    {
        // Works:
        auto groups = records.multiSort!(`a.x < b.x`).groupBy;
        writeln(groups);
    }

    {
        // Doesn't work:
        auto groups = records.multiSort!(`a.x < b.x`, `a.y < b.y`).groupBy;
        writeln(groups);
    }
}

struct Record
{
    int x;
    int y;
}



SortedRange.groupBy apparently delegates the work to chunkBy which chokes on
the predicate defined by multiSort:

/dlang/dmd-beta/linux/bin64/../../src/phobos/std/algorithm/iteration.d(1821):
Error: static assert:  "chunkBy expects either a binary predicate or a unary
predicate on range elements of type: Record"
/dlang/dmd-beta/linux/bin64/../../src/phobos/std/algorithm/iteration.d(1896):  
     instantiated from here: ChunkByImplIsUnary!(__lambda1, Record[])
/dlang/dmd-beta/linux/bin64/../../src/phobos/std/algorithm/iteration.d(2110):  
     instantiated from here: ChunkByImpl!(__lambda1, Record[])
/dlang/dmd-beta/linux/bin64/../../src/phobos/std/range/package.d(10730):       
instantiated from here: chunkBy!((a, b) => !predFun(a, b) && !predFun(b, a),
Record[])
onlineapp.d(21):        instantiated from here: groupBy!()

Commenting out the "doesn't work" section demonstrates that multiSort's
predicate works fine as long there's a single original predicate.

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