http://d.puremagic.com/issues/show_bug.cgi?id=8486

           Summary: Possibly wrong interaction of Variant and const arrays
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: cybevnm@gmail.com


--- Comment #0 from cybevnm <cybevnm@gmail.com> 2012-07-31 12:36:58 PDT ---
During initializing Variant, D discards top level const of array, which leads to little unintuitive behaviour. Consider code:

import std.stdio;
import std.variant;
void main()
{
  const int[] arr;
  Variant v = Variant( arr );
  writeln( v.peek!( typeof( arr ) )() );
  writeln( v.peek!( const(int)[] )() );
  writeln( v.type() );
}

...and output:
%dmd main.d && ./main.d
null
7FFF358AE298
const(int)[]

As you can see peek works successfully not for original array type, but for type without top level const. Is Variant supposed to work in that way ?

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klickverbot <code@klickverbot.at> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |code@klickverbot.at


--- Comment #1 from klickverbot <code@klickverbot.at> 2012-07-31 13:07:32 PDT ---
Variant should generally ignore top-level const for »value types« (i.e. array slices, pointers, integral types, …).

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--- Comment #2 from klickverbot <code@klickverbot.at> 2012-07-31 13:09:26 PDT ---
… the point being that this is, if anything, analogous to the constness of the Variant instance itself, not the contained type.

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--- Comment #3 from cybevnm <cybevnm@gmail.com> 2012-08-01 12:58:14 PDT ---
(In reply to comment #2)
> … the point being that this is, if anything, analogous to the constness of the Variant instance itself, not the contained type.

Unfortunately, Variant's handling of arrays is different to handling of class references. So we have next situtation:

import std.variant;
class C { }
void main()
{
  {
    const C c;
    Variant v = Variant( c );
    assert( v.peek!( typeof(c) )() != null );
  }
  {
    const C[] cs;
    Variant v = Variant( cs );
    assert( v.peek!( typeof(cs) )() == null );
 }
}
But array top-level const cutting is understandable, it's the way it works in
all other places...

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--- Comment #4 from klickverbot <code@klickverbot.at> 2012-08-01 13:08:10 PDT ---
I meant that Variant should regard e.g. const(T[]) and const(T)[] or const(T*)
and const(T)* equivalent, i.e. allow implicit conversion between them.

The fact that const is automatically stripped from arrays is not Variant-specific, template functions generally work like that in D.

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--- Comment #5 from cybevnm <cybevnm@gmail.com> 2012-08-01 13:43:19 PDT ---
(In reply to comment #4)
> I meant that Variant should regard e.g. const(T[]) and const(T)[] or const(T*)
> and const(T)* equivalent, i.e. allow implicit conversion between them.

Agree, it will resolve the problem.

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