From 104b8d8dadeb2cadfa779dc12750ad8d3ed55313 Mon Sep 17 00:00:00 2001
From: Mihail Mihov <mihovmihailp@gmail.com>
Date: Thu, 3 Oct 2024 14:33:00 +0200
Subject: [PATCH] Do not assume index of `derivedFn` and `code` parameters

---
 lib/Differentiator/DiffPlanner.cpp | 70 ++++++++++++++++++------------
 1 file changed, 43 insertions(+), 27 deletions(-)

diff --git a/lib/Differentiator/DiffPlanner.cpp b/lib/Differentiator/DiffPlanner.cpp
index a526a6d58..f926488e2 100644
--- a/lib/Differentiator/DiffPlanner.cpp
+++ b/lib/Differentiator/DiffPlanner.cpp
@@ -190,6 +190,20 @@ namespace clad {
 
     FunctionDecl* replacementFD = OverloadedFD ? OverloadedFD : FD;
 
+    auto codeArgIdx = -1;
+    auto derivedFnArgIdx = -1;
+    auto idx = 0;
+    for (auto* arg : call->arguments()) {
+      if (auto* default_arg_expr = dyn_cast<CXXDefaultArgExpr>(arg)) {
+        std::string argName = default_arg_expr->getParam()->getNameAsString();
+        if (argName == "derivedFn")
+          derivedFnArgIdx = idx;
+        else if (argName == "code")
+          codeArgIdx = idx;
+      }
+      ++idx;
+    }
+
     // Index of "CUDAkernel" parameter:
     int numArgs = static_cast<int>(call->getNumArgs());
     if (numArgs > 4) {
@@ -204,8 +218,6 @@ namespace clad {
       call->setArg(kernelArgIdx, cudaKernelFlag);
       numArgs--;
     }
-    auto codeArgIdx = numArgs - 1;
-    auto derivedFnArgIdx = numArgs - 2;
 
     // Create ref to generated FD.
     DeclRefExpr* DRE =
@@ -221,31 +233,35 @@ namespace clad {
     if (isa<CXXMethodDecl>(DRE->getDecl()))
       DRE->setValueKind(CLAD_COMPAT_ExprValueKind_R_or_PR_Value);
 
-    // Add the "&" operator
-    auto newUnOp =
-        SemaRef.BuildUnaryOp(nullptr, noLoc, UnaryOperatorKind::UO_AddrOf, DRE)
-            .get();
-    call->setArg(derivedFnArgIdx, newUnOp);
-
-    // Update the code parameter.
-    if (CXXDefaultArgExpr* Arg
-        = dyn_cast<CXXDefaultArgExpr>(call->getArg(codeArgIdx))) {
-      clang::LangOptions LangOpts;
-      LangOpts.CPlusPlus = true;
-      clang::PrintingPolicy Policy(LangOpts);
-      Policy.Bool = true;
-
-      std::string s;
-      llvm::raw_string_ostream Out(s);
-      FD->print(Out, Policy);
-      Out.flush();
-
-      StringLiteral* SL = utils::CreateStringLiteral(C, Out.str());
-      Expr* newArg =
-        SemaRef.ImpCastExprToType(SL,
-                                  Arg->getType(),
-                                  CK_ArrayToPointerDecay).get();
-      call->setArg(codeArgIdx, newArg);
+    if (derivedFnArgIdx != -1) {
+      // Add the "&" operator
+      auto newUnOp =
+          SemaRef
+              .BuildUnaryOp(nullptr, noLoc, UnaryOperatorKind::UO_AddrOf, DRE)
+              .get();
+      call->setArg(derivedFnArgIdx, newUnOp);
+    }
+
+    // Update the code parameter if it was found.
+    if (codeArgIdx != -1) {
+      if (auto* Arg = dyn_cast<CXXDefaultArgExpr>(call->getArg(codeArgIdx))) {
+        clang::LangOptions LangOpts;
+        LangOpts.CPlusPlus = true;
+        clang::PrintingPolicy Policy(LangOpts);
+        Policy.Bool = true;
+
+        std::string s;
+        llvm::raw_string_ostream Out(s);
+        FD->print(Out, Policy);
+        Out.flush();
+
+        StringLiteral* SL = utils::CreateStringLiteral(C, Out.str());
+        Expr* newArg =
+            SemaRef
+                .ImpCastExprToType(SL, Arg->getType(), CK_ArrayToPointerDecay)
+                .get();
+        call->setArg(codeArgIdx, newArg);
+      }
     }
   }