__traits getMember is context sensetive?

[JDemler via Digitalmars-d-learn]

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?