WAT: opCmp and opEquals woes

[Jonathan M Davis via Digitalmars-d]

On Friday, 25 July 2014 at 12:08:55 UTC, Jacob Carlborg wrote:
> On 25/07/14 12:09, Jonathan M Davis wrote:
>
>> The _only_ code that would break would be code that's 
>> _already_ broken -
>> code that defines opCmp in a way that's inconsistent with the 
>> default
>> opEquals and then doesn't define opEquals. I see no reason to 
>> worry
>> about making sure that we don't break code that's already 
>> broken.
>
> I see no reason why I should define opEquals when opCmp was 
> used for AA keys. You keep ignoring that argument.

opEquals will now be used for AA keys, not opCmp. That's why git 
master generates errors when you have a struct which defines 
opCmp and not opEquals, and you try and use it as an AA key. It 
was done on the theory that your opEquals and opCmp might not 
match (which would be buggy code to begin with, so it would be 
forcing everyone to change their code just because someone might 
have gotten their opEquals and opCmp wrong).

If we keep the same behavior as 2.065 but still change the AAs to 
use opEquals, then there's no need to define opEquals unless the 
type was buggy and defined opCmp in a way that was inconsistent 
with the default opEquals and then didn't define one which was 
consistent. The code will continue to work.

H.S. Teoh wants to change the default-generated opEquals to be 
equivalent to lhs.opCmp(rhs) == 0 in the case where opCmp is 
defined in order to avoid further breaking the code of folks 
whose code is broken and didn't define opEquals when opCmp didn't 
match the default.

So, if we remove the new check for a user-defined opEquals when 
opCmp is defined, then you don't have to define opEquals. If we 
do what H.S. Teoh suggests, then you'll have to define it if you 
want to avoid the additional checks that opCmp would be doing 
that opEquals wouldn't do, but if you didn't care, then you 
wouldn't. If we leave it as it is in git master, then you'd 
always have to define it if you defined opCmp and wanted to use 
it as an AA key, and since opCmp was used for AA keys before, 
that means that _every_ type which didn't define opEquals but was 
used as an AA key will suddenly have to define opEquals and 
toHash and will thus now be broken. So, the current situation in 
git master is the worst all around.

- Jonathan M Davis