complex numbers with vars

[bgoodri]

The following C++ program compiles but segfaults (with clang++-3.6 and g++-5)

#include <stan/math.hpp>
#include <iostream>
#include <complex>

int main() {
  stan::math::var x = 1;
  std::complex<stan::math::var> z = x;
  std::cout << "imaginary part is " << z.imag().val() << std::endl;
  return 0;
}

I think we should try to prevent that from compiling, or at least dying gracefully, but failing that I guess we should initialize the imaginary part to zero?

[syclik]

This issue was moved to stan-dev/math#123