[Issue 19633] New: Identity expression produces wrong result with interface inheritance

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19633

          Issue ID: 19633
           Summary: Identity expression produces wrong result with
                    interface inheritance
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dhasenan at gmail.com

https://dlang.org/spec/expression.html#identity_expressions says:

"For class objects, identity is defined as the object references are for the
same object. Null class objects can be compared with is." This is one option
for comparing two interface references with `is`.

Continuing on, there are other obviously inapplicable options, leaving just the
default: "For other operand types, identity is defined as being the same as
equality." This is another option.

Test case:
---
interface A {}
interface B : A {}
class C : A, B {}
void main()
{
    A a = new C;
    B b = cast(B)a;
    writeln(a is b);  // false
    writeln(cast(Object)a is cast(Object)b);  // true
    writeln(a == b);  // true
}
---

If the first option is chosen, `a is b` should have the same result as
`cast(Object)a is cast(Object)b`, which is true.

If the second option is chosen, `a is b` should have the same result as `a ==
b`, which is true.

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