Issue with std.typecons.scoped and Interfaces

[Andrew Wiley]

Sorry this is long, but it's a somewhat complicated issue that I think
someone who knows a lot about is() could solve very quickly. I hit this a
while back but didn't figure out exactly what the issue was until today. It
seems that std.typecons.scoped doesn't play nice with interfaces:

scopedtest.d (shortened somewhat):
import std.typecons, std.stdio;

class A {
this() { writeln("A"); }
~this() { writeln("~A"); }
}

interface Bob {}

class ABob : A, Bob {
this() { writeln("ABob"); }
~this() { writeln("~ABob"); }
}

void main() { auto abob = scoped!ABob(); }


compiler output:
$ gdc -o scopedtest scopedtest.d
/usr/include/d2/4.6.0/std/typecons.d:2571: Error: template
std.typecons.destroy(T) if (is(T == class)) does not match any function
template declaration
/usr/include/d2/4.6.0/std/typecons.d:2571: Error: template
std.typecons.destroy(T) if (is(T == class)) cannot deduce template function
from argument types !()(A,Bob)
/usr/include/d2/4.6.0/std/typecons.d:2530: Error: template instance
std.typecons.destroy!(ABob) error instantiating
scopedtest.d:18:        instantiated from here: scoped!(ABob,)
scopedtest.d:18: Error: template instance std.typecons.scoped!(ABob,) error
instantiating


std.typecons.destroy:
/*
  Used by scoped() above.  Calls the destructors of an object
  transitively up the inheritance path, but work properly only if the
  static type of the object (T) is known.
 */
private void destroy(T)(T obj) if (is(T == class))
{
    static if (is(typeof(obj.__dtor())))
    {
        obj.__dtor();
    }
    static if (!is(T == Object) && is(T Base == super))
    {
        Base b = obj;
        destroy(b); // <-- this instantiation is failing
    }
}


So it looks like instead of a single type, we're getting a tuple of some
sort because ABob has multiple "superclasses" ? I haven't played with tuples
enough to know exactly what's going on here.
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