Ordering comparisons

[Andrei Alexandrescu via Digitalmars-d]

While reviewing work on array comparisons, Vladimir found an odd issue:

https://issues.dlang.org/show_bug.cgi?id=17244

Investigation reveals that during array comparison for inequality, 
structs are compared by means of memcmp - even if they don't define 
opCmp, and regardless of the types of their fields. This has obvious 
unpleasant consequences:

* Floating point values don't compare well with memcmp

* A struct S { int x; } compares differently on little endian and big 
endian system (!)

* A struct containing a field that defines opCmp ignores it

* ... and many other nefarious issues

The question is what to do to minimize breakage yet "break the bad 
code". The most backward-compatible solution is to define opCmp 
automatically to do a field-by-field lexicographical comparison. The 
most radical solution is disable ordering comparisons unless explicitly 
implemented by the user.


Andrei