[Issue 12109] New: foreach with class alias this range iteration inconsistency

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12109

           Summary: foreach with class alias this range iteration
                    inconsistency
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: peter.alexander.au at gmail.com


--- Comment #0 from Peter Alexander <peter.alexander.au at gmail.com> 2014-02-08 10:28:34 PST ---
Sorry for the title, hard to explain in a few words:

class A
{
    int[] a = [1, 2, 3];
    alias a this;
}

void main()
{
    import std.stdio, std.array;

    auto a = new A();
    while (!a.empty)
        a.popFront();
    writeln(a.a); // []

    auto b = new A();
    foreach (_; b)
        {}
    writeln(b.a); // [1, 2, 3]
}


The problem: iterating using popFront() manually gives a different result from
using foreach. The foreach iteration doesn't consume the range, while the
popFront/empty does. Presumably the foreach is making a copy of the aliased
array and iterating that instead of the A object directly.

Expected: foreach should be consistent with using popFront/empty manually and
should consume the range, i.e. the final writeln(b.a) should give []

It is very important that these are consistent otherwise algorithms that change
from using popFront/empty to foreach will change behaviour when the user is
using a class with alias this range. This may explain the variations seen in
Issue 9506 between releases.

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