http://d.puremagic.com/issues/show_bug.cgi?id=1900

           Summary: Template overload sets & traits templates
           Product: D
           Version: 2.012
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: wbaxter@gmail.com


I'm not sure this kind of thing is supposed to work with template overload sets, but I hope it is supposed to, as it is a pattern I see often in C++ code. The idea is to define some "Traits" templates that define various compile-time aspects of a particular type.

----
module main;

import std.stdio;
import A;
import B;
import generic;

void main()
{
    AClass a;
    BClass b;

    writefln("Traits!(AClass).name: ", Traits!(AClass).name);
    writefln("Traits!(BClass).name: ", Traits!(BClass).name);
    writefln("Traits!(int).name:    ", Traits!(int).name);

}
----
module A;

class AClass
{

}

template Traits(T : AClass)
{
    enum string name = "AClass";
}

----
module B;

class BClass
{

}

string func(BClass b) {
    return "BClass";
}


template Traits(T : BClass)
{
    enum string name = "BClass";
}

----
module generic;

struct Traits(T)
{
    const string name = "any";
}

----


Right now I get:

main.d(18): template instance Traits is not a template declaration, it is a
overloadset
main.d(18): Error: undefined identifier Traits!(AClass).name

(Also note the grammar error in the message "it is a overload set".)


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http://d.puremagic.com/issues/show_bug.cgi?id=1900





------- Comment #1 from samukha@voliacable.com  2008-08-09 06:20 -------
At least, there should be a way to merge template overload sets as it is the case with function overload sets.

alias A.Traits Traits;
alias B.Traits Traits;
alias generic.Traits Traits;

Error: declaration Traits is already defined

Another use case:
----
template Foo(T)
{
    template Bar(U : T)
    {
    }
}

mixin Foo!(int) A;
mixin Foo!(char) B;

alias Bar!(int) bar; //error
----
Error: template instance Bar is not a template declaration, it is a overloadset


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=1900


Eldar Insafutdinov <e.insafutdinov@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |e.insafutdinov@gmail.com


--- Comment #2 from Eldar Insafutdinov <e.insafutdinov@gmail.com> 2009-11-09 22:34:01 PST ---
Seems like there has not been an update on this one for quite along time. That is obviously a big issue, there should be a way to merge template overload sets, otherwise some great opportunities in using templates are lost. I am using templates in Qt bindings for specifying some compile-time information for types. My usecase is very similar to that provided by Bill. I would really like for this one to be resolved.

Thanks.

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--- Comment #3 from Bill Baxter <wbaxter@gmail.com> 2009-11-10 03:59:01 PST ---
(In reply to comment #2)
> Seems like there has not been an update on this one for quite along time. That
> is obviously a big issue, there should be a way to merge template overload
> sets, otherwise some great opportunities in using templates are lost. I am
> using templates in Qt bindings for specifying some compile-time information for
> types. My usecase is very similar to that provided by Bill. I would really like
> for this one to be resolved.
> Thanks.

I think I ended up working around this by putting the essential information into the classes themselves, and then having one Traits template that covered built-in types.  Another big template did bunches of static structural tests to tease the needed information out of a class.

In my case that was sufficient.  But it's certainly more cumbersome to deal with.   Perhaps you can explain your case in a little more detail and we might be able to come up with some workaround?

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull
           Platform|x86                         |All
         OS/Version|Windows                     |All


--- Comment #4 from Kenji Hara <k.hara.pg@gmail.com> 2013-02-14 08:05:16 PST ---
https://github.com/D-Programming-Language/dmd/pull/1660

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--- Comment #5 from github-bugzilla@puremagic.com 2013-07-16 18:35:45 PDT ---
Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/e59a33f2b639dc9a6eb431ebcf694f533916f038 fix Issue 1900 - Template overload sets & traits templates

https://github.com/D-Programming-Language/dmd/commit/cc6b7372f2efffd4c5c0314373af3b595ba06a2c Merge pull request #1660 from 9rnsr/fix1900

Issue 1900 & 8352 & 9235 - Implement Template Overload Set

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|NEW                         |RESOLVED
         Depends on|                            |10658
         Resolution|                            |FIXED


--- Comment #6 from Kenji Hara <k.hara.pg@gmail.com> 2013-07-17 00:10:39 PDT ---
Template overload set has been implemented in git head.
But, merging overload set by using alias declaration is not yet implemented.
See bug 10658.

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|FIXED                       |


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