fix: codegen for templates with multiple parameter packs

lib/CppInterOp/CppInterOp.cpp

@@ -616,8 +616,6 @@
     Type = Type->getUnqualifiedDesugaredType();
     if (auto* ET = llvm::dyn_cast<EnumType>(Type))
       return ET->getDecl();
-    if (auto* FnType = llvm::dyn_cast<FunctionProtoType>(Type))
-      Type = const_cast<clang::Type*>(FnType->getReturnType().getTypePtr());
     return Type->getAsCXXRecordDecl();
   }
   return 0;
@@ -1689,6 +1687,10 @@
   PrintingPolicy Policy((LangOptions()));
   Policy.Bool = true;               // Print bool instead of _Bool.
   Policy.SuppressTagKeyword = true; // Do not print `class std::string`.
+#if CLANG_VERSION_MAJOR > 16
+  Policy.SuppressElaboration = true;
+#endif
+  Policy.FullyQualifiedName = true;
   return compat::FixTypeName(QT.getAsString(Policy));
 }
 
@@ -2103,6 +2105,30 @@
       std::string template_args = complete_name.substr(idx);
       name = name_without_template_args +
              (template_args.empty() ? "" : " " + template_args);
+
+      // If a template has consecutive parameter packs, then it is impossible to
+      // use the explicit name in the wrapper, since the type deduction is what
+      // determines the split of the packs. Instead, we'll revert to the
+      // non-templated function name and hope that the type casts in the wrapper
+      // will suffice.
+      if (FD->isTemplateInstantiation() && FD->getPrimaryTemplate()) {
+        const FunctionTemplateDecl* FTDecl =
+            llvm::dyn_cast<FunctionTemplateDecl>(FD->getPrimaryTemplate());
+        if (FTDecl) {
+          auto* templateParms = FTDecl->getTemplateParameters();
+          int numPacks = 0;
+          for (size_t iParam = 0, nParams = templateParms->size();
+               iParam < nParams; ++iParam) {
+            if (templateParms->getParam(iParam)->isTemplateParameterPack())
+              numPacks += 1;
+            else
+              numPacks = 0;
+          }
+          if (numPacks > 1) {
+            name = name_without_template_args;
+          }
+        }
+      }
     }
     if (op_flag || N <= 1)
       callbuf << name;
@@ -3422,6 +3448,8 @@
   // This will instantiate tape<T> type and return it.
   SourceLocation noLoc;
   QualType TT = S.CheckTemplateIdType(TemplateName(TemplateD), noLoc, TLI);
+  if (TT.isNull())
+    return nullptr;
 
   // Perhaps we can extract this into a new interface.
   S.RequireCompleteType(fakeLoc, TT, diag::err_tentative_def_incomplete_type);

unittests/CppInterOp/FunctionReflectionTest.cpp

@@ -1969,6 +1969,31 @@ TEST(FunctionReflectionTest, GetFunctionCallWrapper) {
   EXPECT_TRUE(err_msg.find("instantiation with no body") != std::string::npos);
   EXPECT_EQ(instantiation_in_host_callable.getKind(),
             Cpp::JitCall::kUnknown); // expect to fail
+
+  Interp->process(R"(
+    template<typename ...T>
+    struct tuple {};
+
+    template<typename ...Ts, typename ...TTs>
+    void tuple_tuple(tuple<Ts...> one, tuple<TTs...> two) { return; }
+
+    tuple<int, double> tuple_one;
+    tuple<char, char> tuple_two;
+  )");
+
+  unresolved_candidate_methods.clear();
+  Cpp::GetClassTemplatedMethods("tuple_tuple", Cpp::GetGlobalScope(),
+                                unresolved_candidate_methods);
+  EXPECT_EQ(unresolved_candidate_methods.size(), 1);
+
+  Cpp::TCppScope_t tuple_tuple = Cpp::BestOverloadFunctionMatch(
+      unresolved_candidate_methods, {},
+      {Cpp::GetVariableType(Cpp::GetNamed("tuple_one")),
+       Cpp::GetVariableType(Cpp::GetNamed("tuple_two"))});
+  EXPECT_TRUE(tuple_tuple);
+
+  auto tuple_tuple_callable = Cpp::MakeFunctionCallable(tuple_tuple);
+  EXPECT_EQ(tuple_tuple_callable.getKind(), Cpp::JitCall::kGenericCall);
 }
 
 TEST(FunctionReflectionTest, IsConstMethod) {