https://d.puremagic.com/issues/show_bug.cgi?id=11858

           Summary: Comparison of unconnected classes using `is` must be
                    disallowed
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: accepts-invalid
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: verylonglogin.reg@gmail.com


--- Comment #0 from Denis Shelomovskij <verylonglogin.reg@gmail.com> 2014-01-02 19:49:31 MSK ---
Comparison of unconnected classes using `is` is definitely a mistake. If one want to do such raw things for some reason, he has to cast to e.g. `Object` or `void*` first. Currently such construction is allowed even in `@safe` code.

This code should compile:
---
class A { }
class B { }

void main()
{
    A a;
    B b;
    static assert(!__traits(compiles, a is b)); // fails
}
---

As a result of the issue it's to easy to make a mistake like `a.prop1 is b` instead of `a.prop1.owner is b`. Also a refactoring of class hierarchy by segregating subclasses becomes a nightmare as you get no errors from type system in situations when you e.g. forget to change `find` predicate.

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

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


--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2014-01-02 13:10:49 PST ---
I think this would definitely catch some bugs. I wonder what kind of impact it would have on code breakage though (perhaps we would go through a warning phase first).

There is definitely a precedent:

-----
void main()
{
    int* a;
    float* b;
    static assert(!__traits(compiles, a is b)); // ok, not allowed
}
-----

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Stewart Gordon <smjg@iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|accepts-invalid             |
                 CC|                            |smjg@iname.com
           Severity|major                       |enhancement


--- Comment #2 from Stewart Gordon <smjg@iname.com> 2014-01-03 05:53:13 PST ---
This isn't accepts-invalid, because there's no rule against it.  It's an enhancement request.

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Denis Shelomovskij <verylonglogin.reg@gmail.com> changed:

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


--- Comment #3 from Denis Shelomovskij <verylonglogin.reg@gmail.com> 2014-01-05 19:20:19 MSK ---
(In reply to comment #2)
> This isn't accepts-invalid, because there's no rule against it.  It's an enhancement request.

It's still a bug as current dmd behaviour is inconsistent as if both types are qualified the compiler rejects such comparison.

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Stewart Gordon <smjg@iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |spec


--- Comment #4 from Stewart Gordon <smjg@iname.com> 2014-01-05 15:22:40 PST ---
Hmm.  There's a concept of a reinterpret cast with pointers, so an int* and a float* can feasibly be pointing to the same memory address.  Since it's far less likely that this will be done with object references, I see now that it would need to be at least as strict to make sense.

Looking at

http://dlang.org/expression.html#IdentityExpression

there isn't a clear statement of what combinations of types are allowed.  There should be.  As such, this is a spec issue as much as a compiler issue.

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