[SYCL] Forward decl free func kernel template args

clang/lib/Sema/SemaSYCL.cpp

@@ -6929,6 +6929,38 @@ class FreeFunctionPrinter {
   /// \param ParmList The parameter list of the function.
   void printFreeFunctionShim(const FunctionDecl *FD, const unsigned ShimCounter,
                              const std::string &ParmList) {
+    // If a free function kernel is a template, where a template parameter is a
+    // class template instantiation, then we need to forward declare this type
+    // before __sycl_shim.
+    if (FD->getPrimaryTemplate()) {
+      auto TAL = FD->getTemplateSpecializationArgs();
+      for (unsigned i = 0; i < TAL->size(); ++i) {
+        auto arg = TAL->get(i);
+        if (arg.getKind() != clang::TemplateArgument::Type)
+          continue;
+        clang::QualType QT = arg.getAsType().getCanonicalType();
+        auto RT = QT->getAs<clang::RecordType>();
+        if (!RT)
+          continue;
+        const clang::RecordDecl *RD = RT->getDecl();
+        if (RD && RD->isStruct()) {
+          const clang::TagType *TT = QT->getAs<clang::TagType>();
+          if (!TT) {
+            return;
+          };
+          const clang::TagDecl *TD = TT->getDecl()->getFirstDecl();
+          if (!TD) {
+            return;
+          }
+          if (auto *CTD = llvm::dyn_cast<clang::ClassTemplateSpecializationDecl>(TD))
+            if (auto *CTpl = CTD->getSpecializedTemplate())
+              CTpl->getTemplateParameters()->print(O, Context, false);
+
+          O << TD->getKindName() << " " << TD->getName() << ";\n";
+        }
+      }
+    }
+
     // Generate a shim function that returns the address of the free function.
     O << "static constexpr auto __sycl_shim" << ShimCounter << "() {\n";
     O << "  return (void (*)(" << ParmList << "))";