Add custom derivative to DerivativeSet

This is required if the definition of the custom derivative is not found in the current translation unit and is linked in from another.
Adding it to the set of derivatives ensures that the custom derivative is not differentiated again using numerical differentiation due to an unavailable definition.

include/clad/Differentiator/DerivativeBuilder.h

@@ -83,7 +83,7 @@ namespace clad {
     clang::Sema& m_Sema;
     plugin::CladPlugin& m_CladPlugin;
     clang::ASTContext& m_Context;
-    const DerivedFnCollector& m_DFC;
+    DerivedFnCollector& m_DFC;
     clad::DynamicGraph<DiffRequest>& m_DiffRequestGraph;
     std::unique_ptr<utils::StmtClone> m_NodeCloner;
     clang::NamespaceDecl* m_BuiltinDerivativesNSD;
@@ -135,7 +135,7 @@ namespace clad {
 
   public:
     DerivativeBuilder(clang::Sema& S, plugin::CladPlugin& P,
-                      const DerivedFnCollector& DFC,
+                      DerivedFnCollector& DFC,
                       clad::DynamicGraph<DiffRequest>& DRG);
     ~DerivativeBuilder();
     /// Reset the model use for error estimation (if any).

include/clad/Differentiator/DerivedFnCollector.h

@@ -27,6 +27,9 @@ class DerivedFnCollector {
   /// Adds a derived function to the collection.
   void Add(const DerivedFnInfo& DFI);
 
+  /// Adds a function to derivative set.
+  void AddToDerivativeSet(const clang::FunctionDecl* FD);
+
   /// Finds a `DerivedFnInfo` object in the collection that satisfies the
   /// given differentiation request.
   DerivedFnInfo Find(const DiffRequest& request) const;

lib/Differentiator/DerivativeBuilder.cpp

@@ -37,7 +37,7 @@ using namespace clang;
 namespace clad {
 
 DerivativeBuilder::DerivativeBuilder(clang::Sema& S, plugin::CladPlugin& P,
-                                     const DerivedFnCollector& DFC,
+                                     DerivedFnCollector& DFC,
                                      clad::DynamicGraph<DiffRequest>& G)
     : m_Sema(S), m_CladPlugin(P), m_Context(S.getASTContext()), m_DFC(DFC),
       m_DiffRequestGraph(G),
@@ -253,6 +253,17 @@ static void registerDerivative(FunctionDecl* derivedFD, Sema& semaRef) {
 
       OverloadedFn =
           m_Sema.ActOnCallExpr(S, UnresolvedLookup, Loc, MARargs, Loc).get();
+
+      // Add the custom derivative to the set of derivatives.
+      // This is required in case the definition of the custom derivative
+      // is not found in the current translation unit and is linked in
+      // from another translation unit.
+      // Adding it to the set of derivatives ensures that the custom
+      // derivative is not differentiated again using numerical
+      // differentiation due to unavailable definition.
+      if (auto* CE = dyn_cast<CallExpr>(OverloadedFn))
+        if (auto* FD = CE->getDirectCallee())
+          m_DFC.AddToDerivativeSet(FD);
     }
     return OverloadedFn;
   }

lib/Differentiator/DerivedFnCollector.cpp

@@ -8,7 +8,11 @@ void DerivedFnCollector::Add(const DerivedFnInfo& DFI) {
          "`DerivedFnCollector::Add` more than once for the same derivative "
          ". Ideally, we shouldn't do either.");
   m_DerivedFnInfoCollection[DFI.OriginalFn()].push_back(DFI);
-  m_DerivativeSet.insert(DFI.DerivedFn());
+  AddToDerivativeSet(DFI.DerivedFn());
+}
+
+void DerivedFnCollector::AddToDerivativeSet(const clang::FunctionDecl* FD) {
+  m_DerivativeSet.insert(FD);
 }
 
 bool DerivedFnCollector::AlreadyExists(const DerivedFnInfo& DFI) const {

tools/ClangPlugin.h

@@ -170,11 +170,12 @@ class CladTimerGroup {
     bool HandleTopLevelDecl(clang::DeclGroupRef D) override {
       if (D.isSingleDecl())
         if (auto* FD = llvm::dyn_cast<clang::FunctionDecl>(D.getSingleDecl()))
-          if (m_DFC.IsDerivative(FD)) {
-            assert(!m_Multiplexer &&
-                   "Must happen only if we failed to rearrange the consumers");
+          // If we build the derivative in a non-standard (with no Multiplexer)
+          // setup, we exit early to give control to the non-standard setup for
+          // code generation.
+          // FIXME: This should go away if Cling starts using the clang driver.
+          if (!m_Multiplexer && m_DFC.IsDerivative(FD))
             return true;
-          }
 
       HandleTopLevelDeclForClad(D);
       AppendDelayed({CallKind::HandleTopLevelDecl, D});