http://d.puremagic.com/issues/show_bug.cgi?id=2006

           Summary: Appending empty array using ~= doesn't work
           Product: D
           Version: 2.012
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: bartosz@relisoft.com


I have an empty array of arrays of string aas. I append to it an empty array of string. The result should be an array aas of length one.

string [][] aas = [];
aas ~= cast (string []) [];
writefln ("%d", aas.length);
aas = aas ~ cast (string []) [];
writefln ("%d", aas.length);

It works correctly when using assignment and concatenation, but id doesn't when using the combined operator ~=. In the latter case, the length of array remains zero.


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------- Comment #1 from wbaxter@gmail.com  2008-04-18 14:46 -------
That's funny I was just about to file this one myself a couple of days ago, BUT then I decided that it probably isn't really a bug.

[] could be an empty array of any type.  So the compiler is interpreting it as an empty string[][], not an empty string[].  You have a T[], you concat an empty T[],  your original array isn't supposed to change.

I will agree it's not the most useful of behaviors, and the doc could certainly be clearer about what will happen in such a case, but I don't think its actually a bug.  It's just the compiler interpreting [] in a not-so-useful manner.  If you type a literal "~= []" in the code then clearly you want something to happen.

The workaround is to use "~= [[]]" instead.


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smjg@iname.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |smjg@iname.com
           Keywords|                            |wrong-code




------- Comment #2 from smjg@iname.com  2008-11-21 15:08 -------
(In reply to comment #1)
> [] could be an empty array of any type.  So the compiler is interpreting it as an empty string[][], not an empty string[].  You have a T[], you concat an empty T[],  your original array isn't supposed to change.

But it has been explicitly cast to a string[].  How can a CastExpression be of any type other than that to which it is cast?


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------- Comment #3 from wbaxter@gmail.com  2008-12-22 19:34 -------
I just got bitten by this again.

float[][] arr;
arr ~= [1.0]; // ok, adds a new element (an array of length 1).
arr ~= []; // not ok, does nothing. :-(

The last line there does nothing, apparently because the compiler interprets it
to be an array of array that's empty, which is the least useful interpretation.
 So I find it unexpected that the compiler interprets it this way.  Once
again... even though I already ran into it once.  I just forgot because it
seems so silly for the compiler to choose the interpretation that it does.

At the very least I'd like the compiler to generate an error saying it doesn't know how to interpret 'arr ~= []'.


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clugdbug@yahoo.com.au changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|2.012                       |1.028




------- Comment #4 from clugdbug@yahoo.com.au  2009-02-19 07:06 -------
Applies equally to D1.


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Denis Derman <denis.spir@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |denis.spir@gmail.com


--- Comment #5 from Denis Derman <denis.spir@gmail.com> 2011-01-28 06:39:21 PST ---
(In reply to comment #3)
> I just got bitten by this again.
> 
> float[][] arr;
> arr ~= [1.0]; // ok, adds a new element (an array of length 1).
> arr ~= []; // not ok, does nothing. :-(
> 
> The last line there does nothing, apparently because the compiler interprets it
> to be an array of array that's empty, which is the least useful interpretation.
>  So I find it unexpected that the compiler interprets it this way.  Once
> again... even though I already ran into it once.  I just forgot because it
> seems so silly for the compiler to choose the interpretation that it does.
> 
> At the very least I'd like the compiler to generate an error saying it doesn't know how to interpret 'arr ~= []'.

Yes, ambiguity that is bug-prone, because both interpretations can run, must yield compiler error.

Denis

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--- Comment #6 from Stewart Gordon <smjg@iname.com> 2011-10-01 13:13:23 PDT ---
(In reply to comment #5)
> (In reply to comment #3)
>> At the very least I'd like the compiler to generate an error saying it doesn't know how to interpret 'arr ~= []'.
> 
> Yes, ambiguity that is bug-prone, because both interpretations can run, must yield compiler error.

Yes, an uncast [] is ambiguous.  But the original bug report isn't about that, but about

    aas ~= cast (string []) [];

which is completely unambiguous.

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull
            Summary|Empty array literals with   |Empty array literals with
                   |explicit type implicitly    |explicit type implicitly
                   |convert to anything         |convert to any array type


--- Comment #7 from yebblies <yebblies@gmail.com> 2012-02-03 22:16:00 EST ---
https://github.com/D-Programming-Language/dmd/pull/691

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Stewart Gordon <smjg@iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Empty array literals with   |Appending empty array using
                   |explicit type implicitly    |~= cast(T[]) [] doesn't
                   |convert to any array type   |work


--- Comment #8 from Stewart Gordon <smjg@iname.com> 2012-02-03 17:10:48 PST ---
The recent change to the summary line is wrong.

If they merely implicitly converted to any array type, the original code would behave correctly, because the semantic analyser always attempts an exact match before an implicit conversion match.  Clearly something else is going on.

(DMD 2.057 Win32)
----- bz2006.d -----
import std.stdio;

pragma(msg, typeof(cast(string[]) []));

void main() {
    string[][] aas = [];
    aas ~= cast(string[]) [];
    writefln("%d", aas.length);
    aas ~= (string[]).init;
    writefln("%d", aas.length);
    aas = aas ~ cast(string[]) [];
    writefln("%d", aas.length);

    auto empty = cast(string[]) [];
    pragma(msg, typeof(empty));
    aas ~= empty;
    writefln("%d", aas.length);
}
----------
C:\Users\Stewart\Documents\Programming\D\Tests\bugs>dmd bz2006.d
string[]
string[]

C:\Users\Stewart\Documents\Programming\D\Tests\bugs>bz2006
0
1
2
3
----------

Going by this, the problem seems to occur only with this specific representation of an empty array, and only when it is applied directly to ~= and not passed through a variable.

So it isn't an implicit conversion problem.  It would seem that DMD special-cases this form, but this special-casing is broken.

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Appending empty array using |Empty array literals with
                   |~= cast(T[]) [] doesn't     |explicit type implicitly
                   |work                        |convert to any array type


--- Comment #9 from yebblies <yebblies@gmail.com> 2012-02-04 13:10:58 EST ---
No, you're wrong.  I just wrote a patch to fix this bug, I think I know where the problem was and what I had to change to fix it.

eg.
struct S {}

void main()
{
        int[] x = cast(S[])[];
}

It has nothing to do with appending, but the fact that for T[][] the compiler has to choose to append an empty array as a T[] or a T[][], and it does this by checking implicit conversions, which pass incorrectly.

Please at least read the patch before deciding you know what the problem is.

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