Fix dyn_cast assertion failure on empty switch case statements

Fixes #1815

Author: Vedant2005goyal

lib/Differentiator/CladUtils.cpp

@@ -152,10 +152,10 @@ namespace clad {
                                                Stmt* S) {
       llvm::SmallVector<Stmt*, 16> block;
       block.push_back(S);
-      CompoundStmt* CS = dyn_cast<CompoundStmt>(initial);
+      auto* CS = llvm::dyn_cast_or_null<CompoundStmt>(initial);
       if (CS)
         block.append(CS->body_begin(), CS->body_end());
-      else
+      else if (initial)
         block.push_back(initial);
       auto stmtsRef = clad_compat::makeArrayRef(block.begin(), block.end());
       return clad_compat::CompoundStmt_Create(C, stmtsRef /**/CLAD_COMPAT_CLANG15_CompoundStmt_Create_ExtraParam1(CS), noLoc, noLoc);

lib/Differentiator/ReverseModeVisitor.cpp

@@ -2011,9 +2011,12 @@ Expr* ReverseModeVisitor::getStdInitListSizeExpr(const Expr* E) {
     // We need to check if the last parameter is actually a tracker because
     // custom derivatives currently don't have it.
     if (calleeFnForwPassFD) {
-      QualType lastParamType =
-          calleeFnForwPassFD->parameters().back()->getType();
-      usingRestoreTracker = (utils::GetValueType(lastParamType) == trackerType);
+      if (!calleeFnForwPassFD->parameters().empty()) {
+        QualType lastParamType =
+            calleeFnForwPassFD->parameters().back()->getType();
+        usingRestoreTracker =
+            (utils::GetValueType(lastParamType) == trackerType);
+      }
     }
 
     // FIXME: consider moving non-diff analysis to DiffPlanner.

test/Regressions/issue-1815.cpp

@@ -0,0 +1,42 @@
+// RUN: %cladclang -Xclang -plugin-arg-clad -Xclang -fdump-derived-fn -fsyntax-only -std=c++17 -I%S/../../include %s | FileCheck %s
+
+#include "clad/Differentiator/Differentiator.h"
+#include <cstdarg>
+
+typedef enum { a } b;
+typedef enum { c, d } e;
+e f;
+
+int *g() {
+  switch (f) { 
+    case c: 
+      break; 
+  }
+  return nullptr; 
+}
+
+void h(b, e, char[], char[], int, bool, char, char *, va_list) { 
+  g(); 
+}
+
+char i, o, j;
+int k;
+
+void l(float val) {
+  va_list arg;
+  h(a, d, &i, &o, k, 0, '\0', &j, arg);
+}
+
+double n(double x) {
+  l(x);
+  return x * x; 
+}
+
+void check() {
+  auto grad_n = clad::gradient(n);
+}
+
+// CHECK: void n_grad(double x, double *_d_x)
+// CHECK: l(x);
+// CHECK: *_d_x += 1 * x;
+// CHECK: *_d_x += x * 1;