Add a test for #627

This issue was completely fixed after #1619.
Fixes #627.

Author: PetroZarytskyi

test/Gradient/UserDefinedTypes.C

@@ -1296,6 +1296,52 @@ double fn35(double x, double y) {
 // CHECK-NEXT:      _d_ref += 1;
 // CHECK-NEXT:  }
 
+class PrivClass {
+private:
+  size_t n_;
+public:
+  PrivClass(size_t n) : n_(n) {}
+
+  double multiply(double x) {
+    return n_*x;
+  }
+};
+
+// CHECK:  static void constructor_pullback(size_t n, PrivClass *_d_this, size_t *_d_n) {
+// CHECK-NEXT:      {
+// CHECK-NEXT:          *_d_n += _d_this->n_;
+// CHECK-NEXT:          _d_this->n_ = {{0U|0UL|0ULL}};
+// CHECK-NEXT:      }
+// CHECK-NEXT:  }
+
+// CHECK:  void multiply_pullback(double x, double _d_y, PrivClass *_d_this, double *_d_x) {
+// CHECK-NEXT:      {
+// CHECK-NEXT:          _d_this->n_ += _d_y * x;
+// CHECK-NEXT:          *_d_x += this->n_ * _d_y;
+// CHECK-NEXT:      }
+// CHECK-NEXT:  }
+
+double fn36(double x, size_t n){
+  PrivClass E(n);
+  return E.multiply(x); // (x*n)
+}
+
+// CHECK:  void fn36_grad_0(double x, size_t n, double *_d_x) {
+// CHECK-NEXT:      size_t _d_n = {{0U|0UL|0ULL}};
+// CHECK-NEXT:      PrivClass E(n);
+// CHECK-NEXT:      PrivClass _d_E(E);
+// CHECK-NEXT:      clad::zero_init(_d_E);
+// CHECK-NEXT:      {
+// CHECK-NEXT:          double _r1 = 0.;
+// CHECK-NEXT:          E.multiply_pullback(x, 1, &_d_E, &_r1);
+// CHECK-NEXT:          *_d_x += _r1;
+// CHECK-NEXT:      }
+// CHECK-NEXT:      {
+// CHECK-NEXT:          size_t _r0 = {{0U|0UL|0ULL}};
+// CHECK-NEXT:          PrivClass::constructor_pullback(n, &_d_E, &_r0);
+// CHECK-NEXT:          _d_n += _r0;
+// CHECK-NEXT:      }
+// CHECK-NEXT:  }
 void print(const Tangent& t) {
   for (int i = 0; i < 5; ++i) {
     printf("%.2f", t.data[i]);
@@ -1436,4 +1482,9 @@ int main() {
 
     INIT_GRADIENT(fn35);
     TEST_GRADIENT(fn35, /*numOfDerivativeArgs=*/2, -5, 6, &d_i, &d_j);    // CHECK-EXEC: {0.00, 1.00}
+
+    auto fn36_grad = clad::gradient(fn36, "x");
+    d_i = 0;
+    fn36_grad.execute(i, 5, &d_i);
+    printf("{%.2f}\n", d_i);    // CHECK-EXEC: {5.00}
 }