Incorrect gradient for some cases of std::array used in loops

[gojakuch]

reproducer 1:

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

double fn(double x, double y) {
  std::array<double, 3> a;
  a.fill(x);

  for (size_t i = 0; i < 3; ++i) {
    a[i] *= i;
  }

  double res = 0;
  for (size_t i = 0; i < 3; ++i) {
    res += a[i];
  }

  return res;
}

int main(int argc, char* argv[]) {
    double dx, dy;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    dx = 0; dy = 0;
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}

expected output:
9
3 0
output:
9
2 0

a simpler (likely related) reproducer 2:

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

double fn(double x, double y) {
  std::array<double, 3> a;
  a.fill(x);
  for (size_t i = 0; i < 3; ++i) {
    a[i] *= i;
  }
  return a[0];
}

int main(int argc, char* argv[]) {
    double dx, dy;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    dx = 0; dy = 0;
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}

expected output:
0
0 0
output:
0
1 0

note that this works well for c-style arrays:

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

void fill(double t, double * a) {
  for (int i = 0; i < 3; ++i)
    a[i] = t;
}

double fn(double x, double y) {
  double a[3];
  fill(x, a);
  for (size_t i = 0; i < 3; ++i) {
    a[i] *= i;
  }
  return a[0];
}

int main(int argc, char* argv[]) {
    double dx, dy;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    dx = 0; dy = 0;
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}