User-defined "opIs"

[Marco Leise via Digitalmars-d]

I'm against overloading identity checking, too. Instead I
would propose to change its definition to make Proxy structs
work. The rules are currently:

  For class objects, identity is defined as the object
  references are for the same object. Null class objects can
  be compared with is. 

  For struct objects, identity is defined as the bits in the
  struct being identical. 

  For static and dynamic arrays, identity is defined as
  referring to the same array elements and the same number of
  elements. 

  For other operand types, identity is defined as being the
  same as equality. 

If we changed that to:

  A byte for byte comparison of both operands is performed.
  For reference types this is the reference itself.

Breakage is limited to current uses of "is" that clone the
behavior of "==", which a deprecation warning could catch.
And to comparisons of slices with static arrays. (You'd have
to write `dynamicArr is staticArr[]` then.)
This is all comprehensible since you need to be educated about
the difference between value types and reference types anyways.
What it breaks it makes good for by allowing these to compare
equal:

struct CrcProxy { ubyte[4] value; }

CrcProxy crc32a;
ubyte[4] crc32b;

assert (crc32a is crc32b); // Yes, it is byte identical.

If empowering structs to fully emulate built-in types is still
on the agenda it might be the way of least resistance.

-- 
Marco