Fix DVI info for pullback methods

lib/Differentiator/DiffPlanner.cpp

@@ -275,8 +275,25 @@ namespace clad {
   void DiffRequest::UpdateDiffParamsInfo(Sema& semaRef) {
     // Diff info for pullbacks is generated automatically,
     // its parameters are not provided by the user.
-    if (Mode == DiffMode::experimental_pullback)
+    if (Mode == DiffMode::experimental_pullback) {
+      if (!Function->getPreviousDecl())
+        return;
+      const FunctionDecl* FD = Function->getPreviousDecl();
+      // Might need to update DVI args, as they may be pointing to the
+      // declaration parameters, not the definition parameters.
+      for (size_t i = 0, e = DVI.size(), paramIdx = 0;
+           i < e && paramIdx < FD->getNumParams(); ++i) {
+        auto* param = DVI[i].param;
+        while (paramIdx < FD->getNumParams() &&
+               FD->getParamDecl(paramIdx) != param) {
+            ++paramIdx;
+        }
+        if (paramIdx != FD->getNumParams())
+            // Update the parameter to point to the definition parameter.
+            DVI[i].param = Function->getParamDecl(paramIdx);
+      }
       return;
+    }
     DVI.clear();
     auto& C = semaRef.getASTContext();
     const Expr* diffArgs = Args;