[Issue 13179] AA key type TagIndex now requires equality rather than comparison

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13179

--- Comment #11 from Jonathan M Davis <jmdavisProg at gmx.com> ---
(In reply to bearophile_hugs from comment #10)
> (In reply to Jonathan M Davis from comment #9)
> > I would not expect `a == b` to _ever_ be converted to `a.opCmp(b) == 0`.
> > That's what opEquals is for.
> 
> Why? When opCmp returns zero it means ==. What's wrong about using that
> possible return value to test for equality?

It's an unnecessary efficiency hit. The language has opEquals for a reason, and
it's defined by the language to be ==, whereas opCmp is for <, <=, >=, and >.
And code really shouldn't be calling opEquals or opCmp directly except under
very rare circumstances. They're for generating overloaded operators, not for
calling as normal functions.

I can see an argument for just continuing to have the compiler define == when
opCmp is defined in order to avoid having to declare opEquals as well when the
default would be fine, and I can see an argument that opEquals should be
explicitly defined in that case in order to reduce the odds of there being a
bug when someone defined opCmp in a way that was inconsistent with the built in
opEquals. But opEquals is what's supposed to be defining equality. That's what
it's _for_. That's not opCmp's job.

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