Here is a small test case. I get this error on dmd 2.054/053/052/051.
Error vanishes if I replace size_t with ulong on line 6 (inside the is expression).

$ dmd struct.d
Error: undefined identifier size_t
struct.d(11): Error: template instance struct.printFoo!(Foo!(Bar,1)) error instantiating


// struct.d
import std.stdio;
struct Foo(IF, size_t N) {}
interface Bar {}
void printFoo(T)(T foo) {
  static if(is(T L: Foo!(IF, N), IF, size_t N)) {
      writeln("Type: ", T.stringof);
    }
}
void main() {
  Foo!(Bar, 1) foo;
  printFoo(foo);
}