http://d.puremagic.com/issues/show_bug.cgi?id=8074

           Summary: template-mixin example contradicts text
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: websites
        AssignedTo: nobody@puremagic.com
        ReportedBy: tim.dolores@gmail.com


--- Comment #0 from Tim Smith <tim.dolores@gmail.com> 2012-05-09 09:52:31 PDT ---
The text says:

"If two different mixins are put in the same scope, and each define a declaration with the same name, there is an ambiguity error when the declaration is referenced"

However, the example *does* compile with no error, and runs the Bar.func() method. Here is the exact code I'm using:

$ cat mix08.d
import std.stdio, std.exception;

/*
   If two different mixins are put in the same scope, and each define a
   declaration with the same name, there is an ambiguity error when the
   declaration is referenced:
 */

mixin template Foo() {
    int x = 5;
    void func(int x) { writeln("Foo.func(), x = ", x); }
}

mixin template Bar() {
    int x = 4;
    void func() { writeln("Bar.func(), x = ", x); }
}

mixin Foo;
mixin Bar;

void main() {
    //writefln("x = %d", x); // error, x is ambiguous
    /*
       The call to func() is ambiguous because Foo.func and Bar.func are in
       different scopes.
     */
    func();             // error, func is ambiguous
}

$ dmd|grep DMD
DMD64 D Compiler v2.059
$ dmd -w mix08.d
$ ./mix08
Bar.func(), x = 4
$


I don't know if the text is wrong, or if DMD is wrong.

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--- Comment #1 from Tim Smith <tim.dolores@gmail.com> 2012-05-09 09:54:19 PDT ---
I forgot to link to the exact spec page I'm talking about:

http://dlang.org/template-mixin.html

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrei@erdani.com,
                   |                            |andrej.mitrovich@gmail.com


--- Comment #2 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-06 20:22:31 PST ---
I seriously doubt we could implement this now without breaking code. Chances are people are using it to inject multiple function overloads.

I think we'll simply have to edit that part out of the website. Andrei do you agree?

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--- Comment #3 from Andrei Alexandrescu <andrei@erdani.com> 2013-02-07 09:21:17 PST ---
We should fix this, it's clearly a crass error. We can't afford to arbitrarily pick the last lexical declaration in case of an ambiguity.

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--- Comment #4 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-07 09:53:20 PST ---
(In reply to comment #3)
> We should fix this, it's clearly a crass error. We can't afford to arbitrarily pick the last lexical declaration in case of an ambiguity.

But there is no ambiguity, the two functions are:

void func(int x) { writeln("Foo.func(), x = ", x); }
void func() { writeln("Bar.func(), x = ", x); }

Neither of these can hijack one another.

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--- Comment #5 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-07 09:54:00 PST ---
(In reply to comment #4)
> (In reply to comment #3)
> > We should fix this, it's clearly a crass error. We can't afford to arbitrarily pick the last lexical declaration in case of an ambiguity.
> 
> But there is no ambiguity, the two functions are:
> 
> void func(int x) { writeln("Foo.func(), x = ", x); }
> void func() { writeln("Bar.func(), x = ", x); }
> 
> Neither of these can hijack one another.

Edit: Well except the case of taking the address of such a function which could prove problematic.

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--- Comment #6 from Kenji Hara <k.hara.pg@gmail.com> 2013-02-14 09:01:34 PST ---
I think there is no ambiguity for current behavior.

I can explain it based on the 'overload set resolution rule'.

Please regard these mixin declarations as two import statements.

  mixin Foo;
  mixin Bar;
  void main() { func(); }

Can be considered to:

  import foo;
  import bar;
  void main() { func(); }

And each module contains following declarations.

  module foo;  // ---
  int x = 5;
  void func(int x) { writeln("Foo.func(), x = ", x); }

  module bar;  // ---
  int x = 4;
  void func() { writeln("Bar.func(), x = ", x); }

Finally, the 'func()' is resolved to the call of bar.func.

"compiler will resolve an ambiguous mixin-ed symbol by regarding it as overload set.." - This is much reasonable and consistent rule. If 'foo' is called with some arguments but cannot eliminate the ambiguity, it will be error. Otherwise simply will succeed to call.

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Martin Nowak <code@dawg.eu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |code@dawg.eu


--- Comment #7 from Martin Nowak <code@dawg.eu> 2013-05-20 11:34:15 PDT ---
(In reply to comment #6)
> I can explain it based on the 'overload set resolution rule'.

Yes, according to http://dlang.org/template-mixin.html these form an overload set and only one of the sets has a match.

(In reply to comment #5)
> Edit: Well except the case of taking the address of such a function which could > prove problematic.

No problem here, you just have to specify the function type.

void foo() {}
void foo(int) {}

void main()
{
    void function() a1 = &foo;
    void function(int) a2 = &foo;
    auto b1 = cast(void function())&foo;
    auto b2 = cast(void function(int))&foo;
    assert(cast(void*)a1 !is cast(void*)a2);
    assert(a1 is b1);
    assert(a2 is b2);
}

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