Complain if signatures of forward declaration and generated gradient code do not match

tools/ClangPlugin.cpp

@@ -215,6 +215,59 @@ namespace clad {
         }
       }
 
+      // Only verify forward declaration in reverse mode
+      if (request.Mode == DiffMode::reverse) {
+        bool hasForwardDecl = false;
+        FunctionDecl* deriv = nullptr;
+        FunctionDecl* forwdecl = nullptr;
+
+        if (isa<CXXMethodDecl>(DerivativeDecl)) {
+          // Derivative is a class/struct method
+          // Search for forw decl in same scope
+          DeclContextLookupResult DCLR = DerivativeDecl->getParent()->lookup(
+              DerivativeDecl->getDeclName());
+          if (std::distance(DCLR.begin(), DCLR.end()) >= 2) {
+            // Forward declaration found
+            hasForwardDecl = true;
+            deriv = (*DCLR.begin())->getAsFunction();
+            forwdecl = (*(++DCLR.begin()))->getAsFunction();
+          }
+        }
+
+        else {
+          LookupResult P(S, DerivativeDecl->getNameInfo(),
+                         Sema::LookupNameKind::LookupUsingDeclName,
+                         Sema::RedeclarationKind::ForVisibleRedeclaration);
+          S.LookupName(P, S.TUScope);
+
+          if (std::distance(P.begin(), P.end()) >= 2) {
+            // Forward declaration found with possibly mismatched signature
+            hasForwardDecl = true;
+            deriv = (*P.begin())->getAsFunction();
+            forwdecl = (*(++P.begin()))->getAsFunction();
+          }
+        }
+        // Compare signatures including return type
+        if (hasForwardDecl) {
+          assert(
+              deriv->getReturnType() == forwdecl->getReturnType() &&
+              "Forward declaration of a gradient must have void return type");
+
+          auto derivsignature = deriv->parameters();
+          auto forwsignature = forwdecl->parameters();
+          assert(derivsignature.size() == forwsignature.size() &&
+                 "Unequal number of args in forward declaration and generated "
+                 "gradient");
+
+          for (auto it1 = derivsignature.begin(), it2 = forwsignature.begin();
+               it1 != derivsignature.end() && it2 != forwsignature.end();
+               it1++, it2++) {
+            assert((*it1)->getType() == (*it2)->getType() &&
+                   "Types of arguments do not match in forward decl");
+          }
+        }
+      }
+
       if (DerivativeDecl) {
         if (!alreadyDerived) {
           m_DFC.Add(