Add library declaring CEL optional definitions for the C++ type checker.

PiperOrigin-RevId: 682469153

checker/BUILD

@@ -118,13 +118,10 @@ cc_library(
         "//checker/internal:builtins_arena",
         "//common:constant",
         "//common:decl",
-        "//common:memory",
         "//common:type",
         "//internal:status_macros",
         "@com_google_absl//absl/base:no_destructor",
-        "@com_google_absl//absl/base:nullability",
         "@com_google_absl//absl/status",
-        "@com_google_protobuf//:protobuf",
     ],
 )
 
@@ -146,3 +143,40 @@ cc_test(
         "@com_google_absl//absl/status:status_matchers",
     ],
 )
+
+cc_library(
+    name = "optional",
+    srcs = ["optional.cc"],
+    hdrs = ["optional.h"],
+    deps = [
+        ":type_checker_builder",
+        "//base:builtins",
+        "//checker/internal:builtins_arena",
+        "//common:decl",
+        "//common:type",
+        "//internal:status_macros",
+        "@com_google_absl//absl/base:no_destructor",
+        "@com_google_absl//absl/status",
+    ],
+)
+
+cc_test(
+    name = "optional_test",
+    srcs = ["optional_test.cc"],
+    deps = [
+        ":optional",
+        ":standard_library",
+        ":type_check_issue",
+        ":type_checker",
+        ":type_checker_builder",
+        "//base/ast_internal:ast_impl",
+        "//base/ast_internal:expr",
+        "//checker/internal:test_ast_helpers",
+        "//extensions/protobuf:value",
+        "//internal:testing",
+        "@com_google_absl//absl/status:status_matchers",
+        "@com_google_absl//absl/strings",
+        "@com_google_cel_spec//proto/test/v1/proto3:test_all_types_cc_proto",
+        "@com_google_protobuf//:protobuf",
+    ],
+)

checker/internal/BUILD

@@ -27,6 +27,7 @@ cc_library(
         "//extensions/protobuf:ast_converters",
         "//internal:status_macros",
         "//parser",
+        "//parser:options",
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings:string_view",
     ],

checker/internal/test_ast_helpers.cc

@@ -21,6 +21,7 @@
 #include "common/ast.h"
 #include "extensions/protobuf/ast_converters.h"
 #include "internal/status_macros.h"
+#include "parser/options.h"
 #include "parser/parser.h"
 
 namespace cel::checker_internal {
@@ -30,7 +31,10 @@ using ::google::api::expr::parser::Parse;
 
 absl::StatusOr<std::unique_ptr<Ast>> MakeTestParsedAst(
     absl::string_view expression) {
-  CEL_ASSIGN_OR_RETURN(auto parsed, Parse(expression));
+  static ParserOptions options;
+  options.enable_optional_syntax = true;
+  CEL_ASSIGN_OR_RETURN(auto parsed,
+                       Parse(expression, /*description=*/expression, options));
 
   return CreateAstFromParsedExpr(std::move(parsed));
 }

checker/internal/type_check_env.h

@@ -138,6 +138,10 @@ class TypeCheckEnv {
     return functions_.insert({decl.name(), std::move(decl)}).second;
   }
 
+  void InsertOrReplaceFunction(FunctionDecl decl) {
+    functions_[decl.name()] = std::move(decl);
+  }
+
   absl::Nullable<const TypeCheckEnv*> parent() const { return parent_; }
   void set_parent(TypeCheckEnv* parent) { parent_ = parent; }