Fix empty tuple unification

codon/parser/ast/types/class.cpp

@@ -129,12 +129,12 @@ std::string ClassType::realizedTypeName() const {
 }
 
 RecordType::RecordType(Cache *cache, std::string name, std::string niceName,
-                       std::vector<Generic> generics, std::vector<TypePtr> args)
+                       std::vector<Generic> generics, std::vector<TypePtr> args, bool isInternal)
     : ClassType(cache, std::move(name), std::move(niceName), std::move(generics)),
-      args(std::move(args)) {}
+      args(std::move(args)), isInternal(false) {}
 
-RecordType::RecordType(const ClassTypePtr &base, std::vector<TypePtr> args)
-    : ClassType(base), args(std::move(args)) {}
+RecordType::RecordType(const ClassTypePtr &base, std::vector<TypePtr> args, bool isInternal)
+    : ClassType(base), args(std::move(args)), isInternal(isInternal) {}
 
 int RecordType::unify(Type *typ, Unification *us) {
   if (auto tr = typ->getRecord()) {
@@ -157,7 +157,10 @@ int RecordType::unify(Type *typ, Unification *us) {
     }
     // Handle Tuple<->@tuple: when unifying tuples, only record members matter.
     if (startswith(name, TYPE_TUPLE) || startswith(tr->name, TYPE_TUPLE)) {
-      return s1 + int(name == tr->name);
+      if (!args.empty() || (!isInternal && !tr->isInternal)) // prevent int<->() unification
+        return s1 + int(name == tr->name);
+      else
+        return -1;
     }
     return this->ClassType::unify(tr.get(), us);
   } else if (auto t = typ->getLink()) {
@@ -172,7 +175,7 @@ TypePtr RecordType::generalize(int atLevel) {
   auto a = args;
   for (auto &t : a)
     t = t->generalize(atLevel);
-  return std::make_shared<RecordType>(c, a);
+  return std::make_shared<RecordType>(c, a, isInternal);
 }
 
 TypePtr RecordType::instantiate(int atLevel, int *unboundCount,
@@ -182,7 +185,7 @@ TypePtr RecordType::instantiate(int atLevel, int *unboundCount,
   auto a = args;
   for (auto &t : a)
     t = t->instantiate(atLevel, unboundCount, cache);
-  return std::make_shared<RecordType>(c, a);
+  return std::make_shared<RecordType>(c, a, isInternal);
 }
 
 std::vector<TypePtr> RecordType::getUnbounds() const {

codon/parser/ast/types/class.h

@@ -77,13 +77,13 @@ using ClassTypePtr = std::shared_ptr<ClassType>;
 struct RecordType : public ClassType {
   /// List of tuple arguments.
   std::vector<TypePtr> args;
-  char flags = 0;
+  bool isInternal;
 
   explicit RecordType(
       Cache *cache, std::string name, std::string niceName,
       std::vector<ClassType::Generic> generics = std::vector<ClassType::Generic>(),
-      std::vector<TypePtr> args = std::vector<TypePtr>());
-  RecordType(const ClassTypePtr &base, std::vector<TypePtr> args);
+      std::vector<TypePtr> args = std::vector<TypePtr>(), bool isInternal = false);
+  RecordType(const ClassTypePtr &base, std::vector<TypePtr> args, bool isInternal = false);
 
 public:
   int unify(Type *typ, Unification *undo) override;

codon/parser/visitors/typecheck/class.cpp

@@ -26,6 +26,8 @@ void TypecheckVisitor::visit(ClassStmt *stmt) {
   auto typ = Type::makeType(ctx->cache, stmt->name, ctx->cache->rev(stmt->name),
                             stmt->isRecord())
                  ->getClass();
+  if (stmt->isRecord() && stmt->hasAttr(Attr::Internal))
+    typ->getRecord()->isInternal = true;
   if (stmt->isRecord() && startswith(stmt->name, TYPE_PARTIAL)) {
     // Special handling of partial types (e.g., `Partial.0001.foo`)
     if (auto p = in(ctx->cache->partials, stmt->name))