Failure in differentiating class methods with non default constructor

[vaithak]

Minimal example:

# include "clad/Differentiator/Differentiator.h"
# include <iostream>

class Experiment {
private:
  size_t n_;
public:
   Experiment(size_t n) : n_(n) {}

   double multiply(double x) {
     return n_*x;
   }
};

double f(double x, size_t n){
  Experiment E(n);
  return E.multiply(x); // (x*n)
}

int main() {
   size_t n = 3;
   double x = 4;
   double fx = f(x, n);
   std::cout << "f(x, n) = " << fx << "\n";
   
   auto f_dx = clad::gradient(f, "x");
   double dx = 0;
   f_dx.execute(x, n, &dx);
   std::cout << "df/dx = " << dx << "\n";
   return 0;
}

The starting of the generated function of f_grad_0 contains the following snippet:

void f_grad_0(double x, size_t n, clad::array_ref<double> _d_x) {
    size_t _d_n = 0;
    Experiment _d_E(<recovery-expr>({}));
    double _t0;
    Experiment _t1 = <recovery-expr>();
    Experiment E(n);
    _t0 = x;
    _t1 = E;
    ...
}

Where the recovery expressions are because of the following error messages:

error: invalid operands to binary expression ('size_t' (aka 'unsigned long') and 'Experiment')
error: no matching constructor for initialization of 'Experiment'

[vaithak]

One of the root causes for the second error is that some user-defined classes may not have a default constructor, and in reverse mode, we create some global variables at the start of the function and are assigned values later, for ex., in the above example, t1 is declared in the start and assigned later to E.
I can't think of any way other than to explicitly tell the user to make sure that custom classes have a default constructor.

For the first error, still debugging it.