isCallable fails

[frame via Digitalmars-d-learn]

What could cause the following error?

Error: expression isCallable!(data) of type void does not have a 
boolean value


Problematic lines are:


// T         = some class
// S         = "data" (data is member of T)
// typeof(S) = some struct U!X, not void

alias S = __traits(getMember, T, property);

static if (isCallable!S) {
    // some other magic
}


So, struct U has an "alias this", not causing any problems except 
if it calls the getter on type X:

struct U(T) {
  T get() {
        static if (is(T : X)) {
            if (value is null) {
                // load value
            }
        }
        return value;
  }

  T value;
  alias get this;
}


I do not understand why this can affect isCallable as it does 
nothing special. It also compiles if I remove the loading "static 
if" switch.