Fix type unification for type parameters

checker/internal/type_inference_context.cc

@@ -153,6 +153,19 @@ std::optional<Type> WrapperToPrimitive(const Type& t) {
   }
 }
 
+// Tests whether the type contains any type params directly or transitively.
+bool HasTypeParam(const Type& type) {
+  if (type.kind() == TypeKind::kTypeParam) {
+    return true;
+  }
+  for (const auto& param : type.GetParameters()) {
+    if (HasTypeParam(param)) {
+      return true;
+    }
+  }
+  return false;
+}
+
 }  // namespace
 
 Type TypeInferenceContext::InstantiateTypeParams(const Type& type) {
@@ -252,6 +265,26 @@ bool TypeInferenceContext::IsAssignableInternal(
   Type to_subs = Substitute(to, prospective_substitutions);
   Type from_subs = Substitute(from, prospective_substitutions);
 
+  if (from_subs.kind() == TypeKind::kType &&
+      to_subs.kind() == TypeKind::kType) {
+    Type from_inner = from_subs.AsType()->GetType();
+    Type to_inner = to_subs.AsType()->GetType();
+    // If either type contains a type parameter (e.g., type(T) in foo(data,
+    // type(T)) -> T), delegate to inner type unification to bind or validate
+    // type parameter substitutions. Returns true if the inner types
+    // structurally match, unify with an unbound type param, or conform to an
+    // existing binding in 'prospective_substitutions'. Returns false on
+    // structural/kind mismatches (e.g., int vs list(T)), occurs-check cycles,
+    // or conflicting type param bindings.
+    if (HasTypeParam(from_inner) || HasTypeParam(to_inner)) {
+      return IsAssignableInternal(from_inner, to_inner,
+                                  prospective_substitutions);
+    }
+    // Concrete types are coassignable in CEL (e.g., type(1) == type("a"),
+    // type([1]) == list).
+    return true;
+  }
+
   // Types always assignable to themselves.
   // Remainder is checking for assignability across different types.
   if (to_subs == from_subs) {
@@ -313,13 +346,6 @@ bool TypeInferenceContext::IsAssignableInternal(
     }
   }
 
-  if (from_subs.kind() == TypeKind::kType &&
-      to_subs.kind() == TypeKind::kType) {
-    // Types are always assignable to themselves (even if differently
-    // parameterized).
-    return true;
-  }
-
   if (to_subs.kind() == TypeKind::kEnum && from_subs.kind() == TypeKind::kInt) {
     return true;
   }

checker/internal/type_inference_context_test.cc

@@ -846,5 +846,267 @@ TEST(TypeInferenceContextTest, AssignabilityContextReset) {
               IsTypeKind(TypeKind::kDouble));
 }
 
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_ConcreteTypes_Coassignable) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type int_type = TypeType(&arena, IntType());
+  Type string_type = TypeType(&arena, StringType());
+
+  EXPECT_TRUE(context.IsAssignable(int_type, string_type));
+  EXPECT_TRUE(context.IsAssignable(string_type, int_type));
+}
+
+TEST(TypeInferenceContextTest, TypeTypeAssignability_MapContainerErasure) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type map_int_uint = TypeType(&arena, MapType(&arena, IntType(), UintType()));
+  Type map_dyn_dyn = TypeType(&arena, MapType(&arena, DynType(), DynType()));
+
+  EXPECT_TRUE(context.IsAssignable(map_int_uint, map_dyn_dyn));
+}
+
+TEST(TypeInferenceContextTest, TypeTypeAssignability_ListContainerErasure) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type list_int = TypeType(&arena, ListType(&arena, IntType()));
+  Type list_dyn = TypeType(&arena, ListType(&arena, DynType()));
+
+  EXPECT_TRUE(context.IsAssignable(list_int, list_dyn));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_TypeParamTarget_BindsConcreteType) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type type_param_t =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("T")));
+  Type from_type = TypeType(&arena, IntType());
+
+  EXPECT_TRUE(context.IsAssignable(from_type, type_param_t));
+
+  Type resolved_type = context.FinalizeType(type_param_t);
+  ASSERT_THAT(resolved_type, IsTypeKind(TypeKind::kType));
+  EXPECT_THAT(resolved_type.AsType()->GetParameters(),
+              ElementsAre(IsTypeKind(TypeKind::kInt)));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_TypeParamSource_BindsConcreteType) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type type_param_t =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("T")));
+  Type to_type = TypeType(&arena, IntType());
+
+  EXPECT_TRUE(context.IsAssignable(type_param_t, to_type));
+
+  Type resolved_type = context.FinalizeType(type_param_t);
+  ASSERT_THAT(resolved_type, IsTypeKind(TypeKind::kType));
+  EXPECT_THAT(resolved_type.AsType()->GetParameters(),
+              ElementsAre(IsTypeKind(TypeKind::kInt)));
+}
+
+TEST(TypeInferenceContextTest, TypeTypeAssignability_NestedTypeParam_Unifies) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type type_param_t =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("T")));
+  Type type_param_r = context.InstantiateTypeParams(
+      TypeType(&arena, TypeType(&arena, TypeParamType("R"))));
+
+  EXPECT_TRUE(context.IsAssignable(type_param_t, type_param_r));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_DeeplyNestedTypeParam_BindsConcreteType) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type from_type = TypeType(&arena, TypeType(&arena, IntType()));
+  Type to_type = context.InstantiateTypeParams(
+      TypeType(&arena, TypeType(&arena, TypeParamType("T"))));
+
+  EXPECT_TRUE(context.IsAssignable(from_type, to_type));
+
+  Type resolved_type = context.FinalizeType(to_type);
+  ASSERT_THAT(resolved_type, IsTypeKind(TypeKind::kType));
+  Type inner_type = resolved_type.AsType()->GetType();
+  ASSERT_THAT(inner_type, IsTypeKind(TypeKind::kType));
+  EXPECT_THAT(inner_type.AsType()->GetParameters(),
+              ElementsAre(IsTypeKind(TypeKind::kInt)));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_CompositeListTypeParam_BindsConcreteType) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type from_type = TypeType(&arena, ListType(&arena, IntType()));
+  Type to_type = context.InstantiateTypeParams(
+      TypeType(&arena, ListType(&arena, TypeParamType("T"))));
+
+  EXPECT_TRUE(context.IsAssignable(from_type, to_type));
+
+  Type resolved_type = context.FinalizeType(to_type);
+  ASSERT_THAT(resolved_type, IsTypeKind(TypeKind::kType));
+  Type inner_type = resolved_type.AsType()->GetType();
+  ASSERT_THAT(inner_type, IsTypeKind(TypeKind::kList));
+  EXPECT_THAT(inner_type.AsList()->GetElement(), IsTypeKind(TypeKind::kInt));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_CompositeMapTypeParam_BindsConcreteTypes) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type from_type = TypeType(&arena, MapType(&arena, StringType(), IntType()));
+  Type to_type = context.InstantiateTypeParams(TypeType(
+      &arena, MapType(&arena, TypeParamType("K"), TypeParamType("V"))));
+
+  EXPECT_TRUE(context.IsAssignable(from_type, to_type));
+
+  Type resolved_type = context.FinalizeType(to_type);
+  ASSERT_THAT(resolved_type, IsTypeKind(TypeKind::kType));
+  Type inner_type = resolved_type.AsType()->GetType();
+  ASSERT_THAT(inner_type, IsTypeKind(TypeKind::kMap));
+  EXPECT_THAT(inner_type.AsMap()->GetKey(), IsTypeKind(TypeKind::kString));
+  EXPECT_THAT(inner_type.AsMap()->GetValue(), IsTypeKind(TypeKind::kInt));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_OptionalTypeParam_Unifies) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type from_type = TypeType(&arena, OptionalType(&arena, IntType()));
+  Type to_type = context.InstantiateTypeParams(
+      TypeType(&arena, OptionalType(&arena, TypeParamType("T"))));
+
+  EXPECT_TRUE(context.IsAssignable(from_type, to_type));
+
+  Type resolved_type = context.FinalizeType(to_type);
+  ASSERT_THAT(resolved_type, IsTypeKind(TypeKind::kType));
+  Type inner_type = resolved_type.AsType()->GetType();
+  ASSERT_THAT(inner_type, IsTypeKind(TypeKind::kOpaque));
+  EXPECT_THAT(inner_type.AsOpaque()->GetParameters(),
+              ElementsAre(IsTypeKind(TypeKind::kInt)));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_IncompatibleTypeParams_ReturnsFalse) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type from_type = context.InstantiateTypeParams(
+      TypeType(&arena, ListType(&arena, TypeParamType("T"))));
+  Type to_type = TypeType(&arena, IntType());
+
+  EXPECT_FALSE(context.IsAssignable(from_type, to_type));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_ConflictingBoundTypeParam_ReturnsFalse) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type param_t =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("T")));
+  EXPECT_TRUE(context.IsAssignable(TypeType(&arena, StringType()), param_t));
+  EXPECT_FALSE(context.IsAssignable(param_t, TypeType(&arena, IntType())));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_OccursCheck_FailsOnSelfReference) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type param_t =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("T")));
+  Type to_type = TypeType(&arena, param_t);
+
+  EXPECT_FALSE(context.IsAssignable(param_t, to_type));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeAssignability_OccursCheck_FailsOnTransitiveCycle) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  Type param_t =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("T")));
+  Type param_r =
+      context.InstantiateTypeParams(TypeType(&arena, TypeParamType("R")));
+
+  EXPECT_TRUE(context.IsAssignable(param_t, TypeType(&arena, param_r)));
+  EXPECT_FALSE(context.IsAssignable(param_r, TypeType(&arena, param_t)));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeOverloadResolution_TypeParamInTypeType_ResolvesReturnTypeInt) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  ASSERT_OK_AND_ASSIGN(
+      FunctionDecl decl,
+      MakeFunctionDecl("cast",
+                       MakeOverloadDecl("cast_t", TypeParamType("T"), DynType(),
+                                        TypeType(&arena, TypeParamType("T")))));
+
+  std::optional<TypeInferenceContext::OverloadResolution> resolution =
+      context.ResolveOverload(decl, {StringType(), TypeType(&arena, IntType())},
+                              false);
+  ASSERT_TRUE(resolution.has_value());
+  EXPECT_THAT(context.FinalizeType(resolution->result_type),
+              IsTypeKind(TypeKind::kInt));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeOverloadResolution_TypeParamInTypeType_ResolvesReturnTypeString) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  ASSERT_OK_AND_ASSIGN(
+      FunctionDecl decl,
+      MakeFunctionDecl("cast",
+                       MakeOverloadDecl("cast_t", TypeParamType("T"), DynType(),
+                                        TypeType(&arena, TypeParamType("T")))));
+
+  std::optional<TypeInferenceContext::OverloadResolution> resolution =
+      context.ResolveOverload(decl, {IntType(), TypeType(&arena, StringType())},
+                              false);
+  ASSERT_TRUE(resolution.has_value());
+  EXPECT_THAT(context.FinalizeType(resolution->result_type),
+              IsTypeKind(TypeKind::kString));
+}
+
+TEST(TypeInferenceContextTest,
+     TypeTypeOverloadResolution_TypeParamInComposite_ResolvesReturnType) {
+  google::protobuf::Arena arena;
+  TypeInferenceContext context(&arena);
+
+  ASSERT_OK_AND_ASSIGN(
+      FunctionDecl decl,
+      MakeFunctionDecl(
+          "first_elem_type",
+          MakeOverloadDecl(
+              "first_elem_type_overload", TypeParamType("T"), DynType(),
+              TypeType(&arena, ListType(&arena, TypeParamType("T"))))));
+
+  std::optional<TypeInferenceContext::OverloadResolution> resolution =
+      context.ResolveOverload(
+          decl, {StringType(), TypeType(&arena, ListType(&arena, IntType()))},
+          false);
+  ASSERT_TRUE(resolution.has_value());
+  EXPECT_THAT(context.FinalizeType(resolution->result_type),
+              IsTypeKind(TypeKind::kInt));
+}
+
 }  // namespace
 }  // namespace cel::checker_internal

checker/optional_test.cc

@@ -335,5 +335,39 @@ INSTANTIATE_TEST_SUITE_P(
                  "== null",
                  _, "no matching overload for '_==_'"}));
 
+class OptionalListTypePermutationsTest
+    : public testing::TestWithParam<std::string> {};
+
+TEST_P(OptionalListTypePermutationsTest, ResolvesToListDyn) {
+  ASSERT_OK_AND_ASSIGN(
+      std::unique_ptr<TypeCheckerBuilder> builder,
+      CreateTypeCheckerBuilder(GetSharedTestingDescriptorPool()));
+  ASSERT_THAT(builder->AddLibrary(StandardCheckerLibrary()), IsOk());
+  ASSERT_THAT(builder->AddLibrary(OptionalCheckerLibrary()), IsOk());
+  ASSERT_OK_AND_ASSIGN(std::unique_ptr<TypeChecker> checker,
+                       std::move(*builder).Build());
+
+  const std::string& expr = GetParam();
+  ASSERT_OK_AND_ASSIGN(auto ast, MakeTestParsedAst(expr));
+  ASSERT_OK_AND_ASSIGN(auto result, checker->Check(std::move(ast)));
+  EXPECT_THAT(result.GetIssues(), IsEmpty());
+  ASSERT_OK_AND_ASSIGN(auto checked_ast, result.ReleaseAst());
+  EXPECT_EQ(checked_ast->GetTypeOrDyn(checked_ast->root_expr().id()),
+            TypeSpec(ListTypeSpec(std::make_unique<TypeSpec>(DynTypeSpec()))));
+  ASSERT_EQ(checked_ast->root_expr().list_expr().elements().size(), 3);
+  for (const auto& elem : checked_ast->root_expr().list_expr().elements()) {
+    EXPECT_TRUE(checked_ast->GetTypeOrDyn(elem.expr().id()).has_type());
+  }
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    OptionalTests, OptionalListTypePermutationsTest,
+    ::testing::Values("[type([]), int, type(optional.none())]",
+                      "[type([]), type(optional.none()), int]",
+                      "[int, type([]), type(optional.none())]",
+                      "[int, type(optional.none()), type([])]",
+                      "[type(optional.none()), type([]), int]",
+                      "[type(optional.none()), int, type([])]"));
+
 }  // namespace
 }  // namespace cel

checker/standard_library.cc

@@ -112,13 +112,13 @@ Type TypeDynType() {
 
 Type TypeListType() {
   static absl::NoDestructor<Type> kInstance(
-      TypeType(BuiltinsArena(), ListOfA()));
+      TypeType(BuiltinsArena(), ListType(BuiltinsArena(), DynType())));
   return *kInstance;
 }
 
 Type TypeMapType() {
-  static absl::NoDestructor<Type> kInstance(
-      TypeType(BuiltinsArena(), MapOfAB()));
+  static absl::NoDestructor<Type> kInstance(TypeType(
+      BuiltinsArena(), MapType(BuiltinsArena(), DynType(), DynType())));
   return *kInstance;
 }