http://d.puremagic.com/issues/show_bug.cgi?id=2126

           Summary: class template specialization and inheritance
           Product: D
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: www.digitalmars.com
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: michael.kiermaier@gmx.net


The following code

//begin code
import std.stdio;

class A(TT) { }

class B(TT) : A!(TT) { }

class C(T) {
    static void tellMe() {
        writefln("implementation for generic T.");
    }
}

class C(T:A!(TT),TT) {
    static void tellMe() {
        writefln("implememtation for T the same as or derived from A!(TT).");
    }
}

void main() {
    C!(A!(int)).tellMe();
    C!(B!(int)).tellMe();
    C!(int).tellMe();
}
//end code

compiled by DMD v2.008 produces the output

implememtation for T the same as or derived from A!(TT).
implementation for generic T.
implementation for generic T.


According to the documentation, the colon in 'C(T:A!(TT),TT)' means "the same
as or derived from". So the second line of the output should be "implememtation
for T the same as or derived from A!(TT)." in my opinion.


There was a discussion about this issue in digitalmars.D, see http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=71757

~Michael


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=2126





------- Comment #1 from brunodomedeiros+bugz@gmail.com  2008-06-13 16:27 -------
I think this behavior is not a bug, it could be a request for enhancement at most. I'm not sure it's worth it though, since it could be somewhat complicated, and a trivial workaround is available.

First of all, if I got it right, here's why it doesn't work: The thing is, the "TT : A!(TT)" is a partial specialization, and because of that, it behaves differently from a normal specialization, such as ": A" where A where a class or some other type. It has to be able to deduce type TT, and because of that it has to look at the template instance parameter, such as "A!(int)", and see that it is created from template A with parameters (int), and see that it matches "A!(TT)", with TT being int. That is somewhat odd in itself, since almost everywhere in D a template instance is kinda "opaque": the program sees the code that was generated from the template, but you can't see the template or arguments that comprise the instanciation. (so in that sense I'm surprised that the "C!(A!(int))" case even works at all)

But the problem is, when you try to match "B!(int)" with "A!(TT)", the compiler
does not match them, because B is not A. They are different templates. Of
course, we can see that B!(int) derives from A!(int), so in fact "B!(int)"
should match ": A!(TT)". However, that vision is lost to the compiler. It
doesn't attempt to re-match using the supertypes of B!(int).

The semantics of D could be changed to account for this, but again, I'm not sure it's worth it. I find template specialization rules to already be somewhat complicated and hard to understand, and I would prefer things to not become unnecessarily complicated.

Also, there is a trivial workaround, use marker interfaces, to control the specialization, thus using only normal specialization rules:

--- ---

import std.stdio;

interface RawA {}
interface RawB {}

class A(T) : RawA { }

class B(T) : A!(T), RawB { }

class B2 : A!(short) { }

class C(T) {
        pragma(msg, "implementation for generic T.");
    static void tellMe() {
        writefln("implementation for generic T.");
    }
}

class C(T: RawA) {
        pragma(msg, "implementation for T: " ~ (typeof(T)).stringof);
    static void tellMe() {
        writefln("implementation for T the same as or derived from A!(ANY).");
    }
}

void main() {
    C!(A!(int)).tellMe(); // implementation for T: A
    C!(B!(int)).tellMe(); // implementation for T: B
    C!(int).tellMe();     // implementation for generic T.
    C!(B2).tellMe();      // implementation for T: B2
}


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=2126


Don <clugdbug@yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |spec
                 CC|                            |clugdbug@yahoo.com.au
          Component|www.digitalmars.com         |DMD
            Version|unspecified                 |D1 & D2
           Severity|normal                      |enhancement


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