__traits getMember is context sensetive?

[via Digitalmars-d-learn]

On Saturday, 13 June 2015 at 10:01:45 UTC, JDemler wrote:
> Hey,
>
> i am trying to wrap my head around __traits.
>
> One thing i just do not understand is following:
>
> struct S{
>    string member1;
>    int member2;
> }
>
> void main(string[] args)
> {
>    foreach(typeStr; __traits(allMembers, S))
>    {
>      auto tp = __traits(getMember, S, typeStr);
>      static if (__traits(isArithmetic, tp))
>        writeln(typeStr ~ " is Arithmetic");
>    }
> }
>
> Does not compile. "main.d(15): Error: need 'this' for 'member1'
> of type 'string'"
>
> But if the inner part of the foreach-loop is changed to:
>
> static if (__traits(isArithmetic, __traits(getMember, S,
> typeStr)))
>        writeln(typeStr ~ " is Arithmetic");
>
> it compiles and does exactly what i expect it to do.
>
> If i understand it correctly __traits(getMember returns a
> reference to that member, so i get why i shouldn't be able to 
> use
> it with the class instead of an instance of a class.
>
> But why does it work if it is nested inside a __traits call?

Try `alias` instead of `auto`:

     struct S{
        string member1;
        int member2;
     }

     alias I(Args...) = Args;

     void main(string[] args)
     {
        import std.stdio;
        foreach(typeStr; __traits(allMembers, S))
        {
          alias tp = I!(__traits(getMember, S, typeStr));
          static if (__traits(isArithmetic, tp))
            writeln(typeStr ~ " is Arithmetic");
        }
     }

`auto` declares a variable, which in this case will probably 
contain a delegate to that member.

The workaround with `I` is needed because of a syntactic 
limitation: `alias tp = __traits(...);` is currently not allowed 
by the grammar.