http://d.puremagic.com/issues/show_bug.cgi?id=6434

           Summary: opDispatch must be considered before alias this.
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: gor@boloneum.com


--- Comment #0 from Gor Gyolchanyan <gor@boloneum.com> 2011-08-03 06:14:26 PDT ---
When a class or struct has an opDispatch and an alias this, the alias this is considered first and a member access gives "no such property" error:

struct A
{
   Variant i;
   alias i this;

   void opDispatch(string name)()
   {
   }
}

unittest
{
   A a;
   a.weird; // no property 'weird' for type 'VariantN!(maxSize)'
}

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch, rejects-valid
                 CC|                            |k.hara.pg@gmail.com


--- Comment #1 from Kenji Hara <k.hara.pg@gmail.com> 2011-08-15 22:15:40 PDT ---
https://github.com/D-Programming-Language/dmd/pull/315

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Max Samukha <samukha@voliacable.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |samukha@voliacable.com


--- Comment #2 from Max Samukha <samukha@voliacable.com> 2011-08-16 02:39:02 PDT ---
I am not sure. opDispatch is meant to be a fallback mechanism used after all regular lookups fail. For example, one intended use of opDispatch is to dynamically handle calls to members unavailable statically. With "alias this" given the lowest priority, one would need to manually forward calls to inherited members:

struct A
{
   Variant i;
   alias i this;

   auto opDispatch(string name, A...)(auto ref A args)
   {
       static if (isCallableOnI!(name, A)))
           mixin("return i." ~ name ~ "(args);"); // this shouldn't be
necessary
       else
           return dispatchDynamically(name, toVariants(args));
   }
}

So the bug is in opDispatch not being considered after "alias this" lookup fails, and not in the order of lookups.

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--- Comment #3 from Max Samukha <samukha@voliacable.com> 2011-08-16 03:07:32 PDT ---
On the other hand, opDispatch considered first gives much more flexibility. Then, what to do with regular inheritance:

class A
{
    int foo();
}

class B : A
{
   void opDispatch(string name)()
   {
   }
}

auto b = new B;
b.foo();

Should foo be opDispatched?

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--- Comment #4 from Gor Gyolchanyan <gor@boloneum.com> 2011-08-16 03:55:33 PDT ---
I agree. This would make more sense. Let the opDispatch be used after alias this.

(In reply to comment #2)
> I am not sure. opDispatch is meant to be a fallback mechanism used after all regular lookups fail. For example, one intended use of opDispatch is to dynamically handle calls to members unavailable statically. With "alias this" given the lowest priority, one would need to manually forward calls to inherited members:
> 
> struct A
> {
>    Variant i;
>    alias i this;
> 
>    auto opDispatch(string name, A...)(auto ref A args)
>    {
>        static if (isCallableOnI!(name, A)))
>            mixin("return i." ~ name ~ "(args);"); // this shouldn't be
> necessary
>        else
>            return dispatchDynamically(name, toVariants(args));
>    }
> }
> 
> So the bug is in opDispatch not being considered after "alias this" lookup fails, and not in the order of lookups.

I think all VISIBLE members should be used before opDispatch.

(In reply to comment #3)
> On the other hand, opDispatch considered first gives much more flexibility. Then, what to do with regular inheritance:
> 
> class A
> {
>     int foo();
> }
> 
> class B : A
> {
>    void opDispatch(string name)()
>    {
>    }
> }
> 
> auto b = new B;
> b.foo();
> 
> Should foo be opDispatched?

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla@digitalmars.com
         Resolution|                            |FIXED


--- Comment #5 from Walter Bright <bugzilla@digitalmars.com> 2011-08-25 17:38:45 PDT ---
https://github.com/D-Programming-Language/dmd/commit/e3f1fa2d00250d8b94f6ae653c11856a1ea227c0

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pelle.mansson@gmail.com


--- Comment #7 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-23 22:20:39 PDT ---
*** Issue 4224 has been marked as a duplicate of this issue. ***

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--- Comment #8 from Kenji Hara <k.hara.pg@gmail.com> 2011-10-23 22:29:00 PDT ---
Reopened pull request. https://github.com/D-Programming-Language/dmd/pull/349

Points:
1. alias this lookup should be considered first.
   It is the synonym of super-class.

2. If alias this lookup fails, *most derived* opDispatch should be used.

// From
https://github.com/D-Programming-Language/dmd/pull/349#issuecomment-1923037
struct A
{
  int opDispatch(string s)() { return 1; }
}
struct B
{
  A a;
  alias a this;
  int opDispatch(string s)() if (s == "bsfunc") { return 2; }
}
void main()
{
  B b;
  assert(b.bsfunc() == 2);
  assert(b.notfoundanywhere() == 1);  // *1
}

The most derived opDispatch from B is B.opDispatch, then b.xxx is always
transformed to b.opDispatch!("xxx").
Therefore, line *1 doesn't compile, and A.opDispatch is never used.

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SomeDude <lovelydear@mailmetrash.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lovelydear@mailmetrash.com


--- Comment #10 from SomeDude <lovelydear@mailmetrash.com> 2012-04-27 07:48:07 PDT ---
The program in description now compiles and runs on 2.059

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |enhancement


--- Comment #11 from Walter Bright <bugzilla@digitalmars.com> 2013-03-06 23:25:22 PST ---
I agree with Steven. opDispatch should be considered first. The rationale is straightforward - the wrapper gets to decide, not the wrappee, what opDispatch means.

If the wrapper wants to forward to the alias this, it can do so with an appropriate implementation of opDispatch. But if alias this takes precedence, one is stuck.

So, barring a more compelling rationale for alias this overriding, this enhancement should be rejected.

I also marked this as an enhancement.

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