DIP1001: DoExpression

[Timon Gehr via Digitalmars-d]

On 06.09.2016 17:23, Steven Schveighoffer wrote:
> On 9/6/16 10:17 AM, Timon Gehr wrote:
>> On 06.09.2016 16:12, Steven Schveighoffer wrote:
>>>
>>> I'm not sure I agree with the general principal of the DIP though. I've
>>> never liked comma expressions, and this seems like a waste of syntax.
>>> Won't tuples suffice here when they take over the syntax? e.g. (x, y,
>>> z)[$-1]
>>
>> (Does not work if x, y or z is of type 'void'.)
>
> Let's first stipulate that z cannot be void here, as the context is you
> want to evaluate to the result of some expression.
>
> But why wouldn't a tuple of type (void, void, T) be valid?

Because 'void' is special. (Language design error imported from C.)

struct S{
     void x; // does not work.
}

There can be no field (or variables) of type 'void'. (void,void,T) has 
two fields of type 'void'.

Just fixing the limitations is also not really possible, as e.g. void* 
and void[] exploit that 'void' is special and have a non-compositional 
meaning.

> It could also auto-reduce to just (T).
>
> -Steve

That would fix the limitation, but it is also quite surprising behaviour.