Use unique_ptr for ReverseModeVisitor::m_ExternalSource

Replace the raw owning pointer m_ExternalSource with
std::unique_ptr<MultiplexExternalRMVSource>. This makes ownership
explicit and exception-safe, removing the manual delete path in the
destructor. The destructor is now defaulted.

All existing usage sites use either bool-test (if (m_ExternalSource))
or arrow dereference (m_ExternalSource->Method()), both of which work
identically with unique_ptr.

Fixes: #1839

Author: Elvand-Lie

include/clad/Differentiator/ReverseModeVisitor.h

@@ -64,10 +64,10 @@ namespace clad {
     // a separate namespace, as well as add getters/setters function of
     // several private/protected members of the visitor classes.
     friend class ErrorEstimationHandler;
-    // FIXME: Should we make this an object instead of a pointer?
-    // Downside of making it an object: We will need to include
-    // 'MultiplexExternalRMVSource.h' file
-    MultiplexExternalRMVSource* m_ExternalSource = nullptr;
+    // External sources are owned by the visitor and tied to the current
+    // derivative generation run. Keep them in a unique_ptr to avoid manual
+    // delete paths and stale ownership.
+    std::unique_ptr<MultiplexExternalRMVSource> m_ExternalSource;
     llvm::SmallVector<const clang::ParmVarDecl*, 16> m_NonIndepParams;
     /// In addition to a sequence of forward-accumulated Stmts (m_Blocks), in
     /// the reverse mode we also accumulate Stmts for the reverse pass which

lib/Differentiator/ReverseModeVisitor.cpp

@@ -252,16 +252,7 @@ Expr* ReverseModeVisitor::getStdInitListSizeExpr(const Expr* E) {
                                          const DiffRequest& request)
       : VisitorBase(builder, request) {}
 
-  ReverseModeVisitor::~ReverseModeVisitor() {
-    if (m_ExternalSource) {
-      // Inform external sources that `ReverseModeVisitor` object no longer
-      // exists.
-      // FIXME: Make this so the lifetime scope of the source matches.
-      // m_ExternalSource->ForgetRMV();
-      // Free the external sources multiplexer since we own this resource.
-      delete m_ExternalSource;
-    }
-  }
+  ReverseModeVisitor::~ReverseModeVisitor() = default;
 
   DerivativeAndOverload ReverseModeVisitor::Derive() {
     assert(m_DiffReq.Function && "Must not be null.");
@@ -4494,7 +4485,7 @@ Expr* ReverseModeVisitor::getStdInitListSizeExpr(const Expr* E) {
 
   void ReverseModeVisitor::AddExternalSource(ExternalRMVSource& source) {
     if (!m_ExternalSource)
-      m_ExternalSource = new MultiplexExternalRMVSource();
+      m_ExternalSource = std::make_unique<MultiplexExternalRMVSource>();
     source.InitialiseRMV(*this);
     m_ExternalSource->AddSource(source);
   }