Return constant reference on query of member type

Since the `MemberType` function is a query function, it should not have
to clone the type. Note that the function is no longer `noexcept` after
the change. Originally, it returned the `unknown` type when the `id` was
not a member of the record. However, since we cannot make up such an
unknown type as a temporary object and return its reference (undefined
behavior), we have to throw an exception. This is acceptable because the
type checker should already ensure that the member exists, thus querying
with a non-existing `id` indicates some internal errors.

include/type.hpp

@@ -194,10 +194,9 @@ class RecordType : public Type {
       const noexcept = 0;
   /// @brief Checks if `id` is a member of the record type.
   virtual bool IsMember(const std::string& id) const noexcept = 0;
-  /// @return The type of a member in struct or union. The unknown type if the
-  /// `id` is not a member of the record type.
-  virtual std::unique_ptr<Type> MemberType(
-      const std::string& id) const noexcept = 0;
+  /// @return The type of a member in struct or union.
+  /// @throw `std::runtime_error` if the `id` is not a member of the record.
+  virtual const Type& MemberType(const std::string& id) const = 0;
   /// @note Every member in union shares the same index 0.
   /// @return The index of a member in struct or union.
   /// @throw `std::runtime_error` if the `id` is not a member of the record.
@@ -227,8 +226,7 @@ class StructType : public RecordType {
   }
   std::string id() const noexcept override;
   bool IsMember(const std::string& id) const noexcept override;
-  std::unique_ptr<Type> MemberType(
-      const std::string& id) const noexcept override;
+  const Type& MemberType(const std::string& id) const override;
   std::size_t MemberIndex(const std::string& id) const override;
   std::size_t OffsetOf(const std::string& id) const override;
   std::size_t OffsetOf(std::size_t index) const override;
@@ -260,8 +258,7 @@ class UnionType : public RecordType {
   }
   std::string id() const noexcept override;
   bool IsMember(const std::string& id) const noexcept override;
-  std::unique_ptr<Type> MemberType(
-      const std::string& id) const noexcept override;
+  const Type& MemberType(const std::string& id) const override;
   std::size_t MemberIndex(const std::string& id) const override;
   std::size_t OffsetOf(const std::string& id) const override;
   std::size_t OffsetOf(std::size_t index) const override;

src/llvm_ir_generator.cpp

@@ -671,7 +671,7 @@ void LLVMIRGenerator::Visit(const RecordMemExprNode& mem_expr) {
   auto res_addr = builder_.CreateStructGEP(
       llvm_type, base_addr, record_type->MemberIndex(mem_expr.id));
   auto res_val = builder_.CreateLoad(
-      builder_helper_.GetLLVMType(*(record_type->MemberType(mem_expr.id))),
+      builder_helper_.GetLLVMType(record_type->MemberType(mem_expr.id)),
       res_addr);
   val_to_id_addr[res_val] = res_addr;
   val_recorder.Record(res_val);

src/type.cpp

@@ -171,15 +171,14 @@ bool StructType::IsMember(const std::string& id) const noexcept {
   return false;
 }
 
-std::unique_ptr<Type> StructType::MemberType(
-    const std::string& id) const noexcept {
+const Type& StructType::MemberType(const std::string& id) const {
   for (const auto& field : fields_) {
     if (field->id == id) {
-      return field->type->Clone();
+      return *field->type;
     }
   }
 
-  return std::make_unique<PrimType>(PrimitiveType::kUnknown);
+  throw std::runtime_error{"member not found in struct!"};
 }
 
 std::size_t StructType::MemberIndex(const std::string& id) const {
@@ -277,15 +276,14 @@ bool UnionType::IsMember(const std::string& id) const noexcept {
   return false;
 }
 
-std::unique_ptr<Type> UnionType::MemberType(
-    const std::string& id) const noexcept {
+const Type& UnionType::MemberType(const std::string& id) const {
   for (const auto& field : fields_) {
     if (field->id == id) {
-      return field->type->Clone();
+      return *field->type;
     }
   }
 
-  return std::make_unique<PrimType>(PrimitiveType::kUnknown);
+  throw std::runtime_error{"member not found in union!"};
 }
 
 std::size_t UnionType::MemberIndex(const std::string& id) const {

src/type_checker.cpp

@@ -497,7 +497,7 @@ void TypeChecker::Visit(RecordMemExprNode& mem_expr) {
   if (auto* record_type =
           dynamic_cast<RecordType*>((mem_expr.expr->type).get())) {
     if (record_type->IsMember(mem_expr.id)) {
-      mem_expr.type = record_type->MemberType(mem_expr.id);
+      mem_expr.type = record_type->MemberType(mem_expr.id).Clone();
     } else {
       assert(false);
       // TODO: Throw error if mem_expr.id is not a symbol's member.