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digitalmars.D.bugs - [Issue 4088] New: opEquals not called on interfaces

reply d-bugmail puremagic.com writes:
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           Summary: opEquals not called on interfaces
           Product: D
           Version: 2.041
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: blocker
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: schveiguy yahoo.com



16:33:39 PDT ---
Given an interface that defines opEquals, the compiler will prefer to call the
object.opEquals(Object, Object) on it.  However, interfaces don't implicitly
cast to Objects thanks to COM.

example:

interface I
{
    bool opEquals(I other);
}

bool foo(I i1, I i2)
{
    return i1 == i2;
}

testopequals.d(8): Error: function object.opEquals (Object lhs, Object rhs) is
not callable using argument types (I,I)
testopequals.d(8): Error: cannot implicitly convert expression (i1) of type
testopequals.I to object.Object
testopequals.d(8): Error: cannot implicitly convert expression (i2) of type
testopequals.I to object.Object

And can someone add 2.042 and 2.043 to the version list? This is tested on
2.043.

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Apr 13 2010
next sibling parent d-bugmail puremagic.com writes:
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15:27:58 PDT ---
Note, this is a problem for dcollections 2.0:

http://www.dsource.org/projects/dcollections/ticket/4

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May 19 2010
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Created an attachment (id=907)
Proposed fix

It is actually a very surprising bug, that interfaces can't be compared as it
might break structs due to the compiler generated opEquals.
The attached patch does an explicit cast to Object.
This would still not work with C++/COM interfaces.

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Trass3r <mrmocool gmx.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mrmocool gmx.de



Can't dmd just check the "type" of a given interface and only allow implicit
casts to Object if it is a normal one, i.e. no COM and no C++ interface?

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08:33:19 PST ---


 Can't dmd just check the "type" of a given interface and only allow implicit
 casts to Object if it is a normal one, i.e. no COM and no C++ interface?


I'm not familiar with the internals of the compiler, but I believe this is
true.  I think it should be statically verifiable that an interface is a COM
interface, and then opEquals can be handled differently.

However, we continue to get silence from Walter on this...

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Trass3r <mrmocool gmx.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid



I think this should be tackled at a deeper level, probably somewhere around
MATCH TypeClass::implicitConvTo(Type *to) in mtype.c

to allow implicit conversions of interfaces to Object in general.

Then also this works:

void foo(Object o) {}
Interface i;

foo(i);
// or 
Object o = i;

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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |benoit tionex.de



*** Issue 2794 has been marked as a duplicate of this issue. ***

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Jun 10 2011
prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com



16:25:25 PDT ---
I'm not at all sure the patch is the right solution to this.

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20:34:51 PDT ---
Issues that need to be resolved:

1. what should happen if this is called with a COM object?

2. how does an opEquals defined in an interface interact with the
object.opEquals?

3. a forced cast, unlike an implicit cast, is a blunt instrument that can do a
lot more than simply cast an interface to its base class. If the arguments are
other types, what are the conseqences of this forced cast?

A change of this sort needs to resolve these issues, and have test cases to
verify them.

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05:47:06 PDT ---


 Issues that need to be resolved:
 
 1. what should happen if this is called with a COM object?


Compiler error.


 2. how does an opEquals defined in an interface interact with the
 object.opEquals?


If the notion of COM interfaces is specialized, then standard interfaces do not
need to define opEquals, it's assumed that any standard interface (not C++ or
COM) derives from Object, and implicit dynamic casting to Object to do opEquals
should work.


 3. a forced cast, unlike an implicit cast, is a blunt instrument that can do a
 lot more than simply cast an interface to its base class. If the arguments are
 other types, what are the conseqences of this forced cast?


An implicit cast to Object would be the best remedy.  However, the issue of
dynamic casting and blunt casting being conflated would be a good issue to
solve too.

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---
https://github.com/D-Programming-Language/druntime/pull/72



 1. what should happen if this is called with a COM object?


If two interfaces are identity comparable, returns its result. Otherwise,
downcast to Object, and the result is false.
And this rule also appleied to C++ interface (COM interface is C++ interface,
and the downcasting from C++ interface to Object returns always null).


 2. how does an opEquals defined in an interface interact with the
 object.opEquals?


An interface's opEquals is not used directly. Object.opEquals is always used
for the interface comparison when it is possible.


 3. a forced cast, unlike an implicit cast, is a blunt instrument that can do a
 lot more than simply cast an interface to its base class. If the arguments are
 other types, what are the conseqences of this forced cast?




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18:42:42 PST ---
This is also the root cause of bug 7451.

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dawg dawgfoto.de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dawg dawgfoto.de




One point of it is to prefer interface opEquals over a downcast.

Also, if an interface had an opEquals with a different signature, then
Object.opEquals becomes hidden.

interface IA
{
    bool opEquals(IA o);
}

class A
{
    bool opEquals(IA o) { return false; }
}

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Walter Bright <bugzilla digitalmars.com> changed:

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



19:11:04 PST ---
*** Issue 7451 has been marked as a duplicate of this issue. ***


Commit pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/9c8b88ca89afef97a5d3b81ba7bd65cac71fd6d0
fix Issue 4088 - opEquals not called on interfaces

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Walter Bright <bugzilla digitalmars.com> changed:

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



19:11:04 PST ---
*** Issue 7451 has been marked as a duplicate of this issue. ***


Commit pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/9c8b88ca89afef97a5d3b81ba7bd65cac71fd6d0
fix Issue 4088 - opEquals not called on interfaces

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Walter Bright <bugzilla digitalmars.com> changed:

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


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dawg dawgfoto.de changed:

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



Reopened because now implicit interface comparison is fixed,
but having opEquals in interfaces still doesn't work.

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20:16:23 PST ---
Unless the checked-in fix doesn't allow two interfaces to compare, this bug
should be closed.  It is about the difference between comparing two objects and
comparing two interfaces.  Prior to this fix, you con't compare two interfaces
*period*, even if they defined an identical signature to Object.opEquals.

I feel that it should be possible to specialize opEquals for interfaces and
objects, but this is a separate problem (and actually an enhancement).  IIUC,
the applied fix makes it so Objects and interfaces compare in the same way.

If I find I can compare two interfaces in the same way I can compare two
objects, I'll close this as resolved, and you may open a different bug
regarding adding the enhancement of overriding the default behavior of
object.opEquals.

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Walter Bright <bugzilla digitalmars.com> changed:

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



22:36:28 PST ---
If there's another issue, please open a new bug report, and please provide an
example of the failing code.

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Steven Schveighoffer <schveiguy yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |FIXED



05:36:21 PST ---
Restoring resolution, this was fixed after all.

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