Add support for declaring an overall expected type to the type checker.

checker/internal/type_check_env.h

@@ -107,6 +107,10 @@ class TypeCheckEnv {
     container_ = std::move(container);
   }
 
+  void set_expected_type(const Type& type) { expected_type_ = std::move(type); }
+
+  const absl::optional<Type>& expected_type() const { return expected_type_; }
+
   absl::Span<const std::unique_ptr<TypeIntrospector>> type_providers() const {
     return type_providers_;
   }
@@ -198,6 +202,8 @@ class TypeCheckEnv {
 
   // Type providers for custom types.
   std::vector<std::unique_ptr<TypeIntrospector>> type_providers_;
+
+  absl::optional<Type> expected_type_;
 };
 
 }  // namespace cel::checker_internal

checker/internal/type_checker_impl.cc

@@ -239,7 +239,7 @@ absl::StatusOr<AstType> FlattenType(const Type& type) {
       return absl::InternalError(
           absl::StrCat("Unsupported type: ", type.DebugString()));
   }
-}  // namespace
+}
 
 class ResolveVisitor : public AstVisitorBase {
  public:
@@ -322,6 +322,13 @@ class ResolveVisitor : public AstVisitorBase {
 
   const absl::Status& status() const { return status_; }
 
+  void AssertExpectedType(const Expr& expr, const Type& expected_type) {
+    Type observed = GetTypeOrDyn(&expr);
+    if (!inference_context_->IsAssignable(observed, expected_type)) {
+      ReportTypeMismatch(expr.id(), expected_type, observed);
+    }
+  }
+
  private:
   struct ComprehensionScope {
     const Expr* comprehension_expr;
@@ -1231,6 +1238,10 @@ absl::StatusOr<ValidationResult> TypeCheckerImpl::Check(
   AstTraverse(ast_impl.root_expr(), visitor, opts);
   CEL_RETURN_IF_ERROR(visitor.status());
 
+  if (env_.expected_type().has_value()) {
+    visitor.AssertExpectedType(ast_impl.root_expr(), *env_.expected_type());
+  }
+
   // If any issues are errors, return without an AST.
   for (const auto& issue : issues) {
     if (issue.severity() == Severity::kError) {

checker/internal/type_checker_impl.h

@@ -33,7 +33,7 @@ namespace cel::checker_internal {
 // See cel::TypeCheckerBuilder for constructing instances.
 class TypeCheckerImpl : public TypeChecker {
  public:
-  TypeCheckerImpl(TypeCheckEnv env, CheckerOptions options = {})
+  explicit TypeCheckerImpl(TypeCheckEnv env, CheckerOptions options = {})
       : env_(std::move(env)), options_(options) {}
 
   TypeCheckerImpl(const TypeCheckerImpl&) = delete;

checker/internal/type_checker_impl_test.cc

@@ -1230,6 +1230,48 @@ TEST(TypeCheckerImplTest, TypeInferredFromStructCreation) {
               std::make_unique<AstType>(ast_internal::DynamicType())))))));
 }
 
+TEST(TypeCheckerImplTest, ExpectedTypeMatches) {
+  google::protobuf::Arena arena;
+  TypeCheckEnv env(GetSharedTestingDescriptorPool());
+
+  env.set_expected_type(MapType(&arena, StringType(), StringType()));
+
+  TypeCheckerImpl impl(std::move(env));
+  ASSERT_OK_AND_ASSIGN(auto ast, MakeTestParsedAst("{}"));
+  ASSERT_OK_AND_ASSIGN(ValidationResult result, impl.Check(std::move(ast)));
+
+  ASSERT_OK_AND_ASSIGN(std::unique_ptr<Ast> checked_ast, result.ReleaseAst());
+
+  const auto& ast_impl = AstImpl::CastFromPublicAst(*checked_ast);
+
+  EXPECT_THAT(
+      ast_impl.type_map(),
+      Contains(Pair(
+          ast_impl.root_expr().id(),
+          Eq(AstType(ast_internal::MapType(
+              std::make_unique<AstType>(ast_internal::PrimitiveType::kString),
+              std::make_unique<AstType>(
+                  ast_internal::PrimitiveType::kString)))))));
+}
+
+TEST(TypeCheckerImplTest, ExpectedTypeDoesntMatch) {
+  google::protobuf::Arena arena;
+  TypeCheckEnv env(GetSharedTestingDescriptorPool());
+
+  env.set_expected_type(MapType(&arena, StringType(), StringType()));
+
+  TypeCheckerImpl impl(std::move(env));
+  ASSERT_OK_AND_ASSIGN(auto ast, MakeTestParsedAst("{'abc': 123}"));
+  ASSERT_OK_AND_ASSIGN(ValidationResult result, impl.Check(std::move(ast)));
+
+  EXPECT_FALSE(result.IsValid());
+  EXPECT_THAT(
+      result.GetIssues(),
+      Contains(IsIssueWithSubstring(
+          Severity::kError,
+          "expected type 'map<string, string>' but found 'map<string, int>'")));
+}
+
 TEST(TypeCheckerImplTest, ContainerLookupForMessageCreation) {
   TypeCheckEnv env(GetSharedTestingDescriptorPool());
   env.set_container("google.protobuf");

checker/type_checker_builder.cc

@@ -32,6 +32,7 @@
 #include "checker/internal/type_checker_impl.h"
 #include "checker/type_checker.h"
 #include "common/decl.h"
+#include "common/type.h"
 #include "common/type_introspector.h"
 #include "internal/status_macros.h"
 #include "internal/well_known_types.h"
@@ -170,4 +171,8 @@ void TypeCheckerBuilder::set_container(absl::string_view container) {
   env_.set_container(std::string(container));
 }
 
+void TypeCheckerBuilder::SetExpectedType(const Type& type) {
+  env_.set_expected_type(type);
+}
+
 }  // namespace cel

checker/type_checker_builder.h

@@ -30,6 +30,7 @@
 #include "checker/internal/type_check_env.h"
 #include "checker/type_checker.h"
 #include "common/decl.h"
+#include "common/type.h"
 #include "common/type_introspector.h"
 #include "google/protobuf/descriptor.h"
 
@@ -91,6 +92,8 @@ class TypeCheckerBuilder {
   absl::Status AddVariable(const VariableDecl& decl);
   absl::Status AddFunction(const FunctionDecl& decl);
 
+  void SetExpectedType(const Type& type);
+
   // Adds function declaration overloads to the TypeChecker being built.
   //
   // Attempts to merge with any existing overloads for a function decl with the