Using __traits(getMember...) in alias statement

[Timon Gehr]

On 11/17/2011 06:41 PM, Timon Gehr wrote:
> On 11/17/2011 06:12 PM, Tobias Pankrath wrote:
>> It would be cool, if the following would be possible.
>> ----
>> immutable string MemberID = "M";
>>
>> struct A {}
>> struct B {
>> alias A M;
>> }
>>
>> template Member(T)
>> {
>> static if(__traits(hasMember, T, MemberID))
>> {
>> alias __traits(getMember, T, MemberID) Member;
>> }
>> else
>> alias TypeTuple!() Member;
>> }
>>
>> void main()
>> {
>> alias Member!(B) M;
>> }
>> ----
>>
>> Currently dmd will print "basic type expected, not __traits".
>> Why isn't traits allowed here and if we allow __traits there, would this
>> introduce any ambiguities?
>>
>
> This works:
>
> alias TypeTuple!(__traits(getMember, T, MemberID)) Member;

This only works if the __traits gets you a TypeTuple back. (It won't 
work in this case, it will create a one-element tuple). Better use ID 
from below.


>
> It is a mere grammar issue. The aliased symbol must look like a valid
> type. I'd like that to change too.
>
> This helps a lot with the current state of affairs:
>
> template ID(alias x){alias x ID;}
>
>
> It will even allow funny things like this:
>
> alias ID!((a,b){return a+b;}) add;
>
> static assert(add(1,2) == 3);