Getting only the data members of a type

[Ali Çehreli]

How can I determine just the data members of a struct or class, 
excluding the member functions? isCallable() comes short as member 
variable that have opCall() defined are also callable:

import std.stdio;
import std.traits;

struct M
{
     void opCall()
     {}
}

struct S
{
     int i;
     M m;

     void foo()
     {}
}

void main()
{
     foreach (member; __traits(allMembers, S)) {
         if (isCallable!(mixin("S." ~ member))) {
             writefln("%5s is a member function", member);

         } else {
             writefln("%5s is a member variable", member);
         }
     }
}

The output shows that S.m is a function but it is not:

     i is a member variable
     m is a member function     <-- unexpected
   foo is a member function

Wait! I found a solution before sending this message. :P isIntegral() works:

         if (isIntegral!(typeof(mixin("S." ~ member)))) {

Now the output is:

     i is a member function
     m is a member variable     <-- good but dubious
   foo is a member variable

The reason is, isIntegral() specifically mentions built in types:

   http://dlang.org/phobos/std_traits.html#isIntegral

<quote>
Detect whether T is a built-in integral type. Types bool, char, wchar, 
and dchar are not considered integral.
</quote>

Thank you,
Ali