[Issue 16657] alias this interacts with generated opCmp and opEquals

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16657

--- Comment #5 from Eyal <eyal at weka.io> ---
We had something like this:

    if(x != y) {
      x = y; // expensive assignment
    }
    // assume all x fields equal y fields here, we assign them all -- ouch!

The semantics now are very surprising:

  x = y // assigns the whole of "x", not just the "alias this" part
  x == y // compares just the "alias this" part, wat?

Well-behaved assignments and equalities should apply to the same part of the
object. That the auto-generated assignment/equals code is NOT well-behaved is
surely to be considered a bug.

Implementation-wise:

It is quite simple to always auto-generate opEquals that compares all fields
with == recursively calling opEquals. The auto-generated opEquals overrides the
"alias this" opEquals for the (Subtype,Subtype) case. I don't see where the
implementation difficulty comes in here.

In Walter's example, auto-gen'd C.opEquals would use == on both fields,
implicitly invoking opEquals of A. No special cases or exceptions.

--