Patch for the failure of functions with void return type and with ret…

…urn statement

Prior to this patch, clad would throw an assertion error when attempting to differentiate void functions that contain a return statement.
The assertion error would indicate that there was an attempt to differentiate a nullptr.
This Patch checks if the there is an expression associsted with the null pointer and only if there is one, we differentiate it.

Added tests for the same

This patch fixes #432

lib/Differentiator/ForwardModeVisitor.cpp

@@ -762,6 +762,10 @@ namespace clad {
   }
 
   StmtDiff ForwardModeVisitor::VisitReturnStmt(const ReturnStmt* RS) {
+    //If there is no return value, we must not attempt to differentiate
+    if (!RS->getRetValue())
+      return nullptr;
+
     StmtDiff retValDiff = Visit(RS->getRetValue());
     Stmt* returnStmt = nullptr;
     if (m_Mode == DiffMode::forward) {

test/ForwardMode/MemberFunctions.C

@@ -28,6 +28,27 @@ public:
   // CHECK-NEXT:     return (_d_this->x + _d_this->y) * i + _t0 * _d_i + (_d_i * j + i * _d_j) * j + _t1 * _d_j;
   // CHECK-NEXT: }
 
+  void mem_fn_with_void_return() {
+	  return;
+  }
+
+  // CHECK: void mem_fn_with_void_return_pushforward(SimpleFunctions *_d_this) {
+  // CHECK-NEXT:}
+
+  double mem_fn_with_void_function_call(double i, double j) {
+    mem_fn_with_void_return();
+    return i*j;
+  } 
+
+  // CHECK:  double mem_fn_with_void_function_call_darg0(double i, double j) {
+  // CHECK-NEXT:     double _d_i = 1;  
+  // CHECK-NEXT:     double _d_j = 0; 
+  // CHECK-NEXT:     SimpleFunctions _d_this_obj;
+  // CHECK-NEXT:     SimpleFunctions *_d_this = &_d_this_obj; 
+  // CHECK-NEXT:     this->mem_fn_with_void_return_pushforward(_d_this);  
+  // CHECK-NEXT:     return _d_i * j + i * _d_j;  
+  // CHECK-NEXT:}
+
   double mem_fn_with_var_arg_list(double i, double j, ...)  { 
     return (x+y)*i + i*j*j; 
   } 
@@ -727,6 +748,8 @@ int main() {
   TEST(mem_fn, 3, 5)  // CHECK-EXEC: 30.00 
                       // CHECK-EXEC: 33.00 
 
+  TEST(mem_fn_with_void_function_call, 3, 5) //CHECK-EXEC: 5.00
+
   TEST(mem_fn_with_var_arg_list, 3, 5)  // CHECK-EXEC: 30.00 
                                         // CHECK-EXEC: 33.00