Mock default enabling recursive planning for identifying compatibility issues.

common/value.cc

@@ -270,15 +270,12 @@ bool Value::IsZeroValue() const {
 }
 
 std::ostream& operator<<(std::ostream& out, const Value& value) {
-  value.AssertIsValid();
   return absl::visit(
       [&out](const auto& alternative) -> std::ostream& {
         if constexpr (std::is_same_v<
                           absl::remove_cvref_t<decltype(alternative)>,
                           absl::monostate>) {
-          // In optimized builds, we do nothing. In debug builds we cannot
-          // reach here.
-          return out;
+          return out << "default ctor Value";
         } else {
           return out << alternative;
         }
@@ -513,15 +510,12 @@ bool ValueView::IsZeroValue() const {
 }
 
 std::ostream& operator<<(std::ostream& out, ValueView value) {
-  value.AssertIsValid();
   return absl::visit(
       [&out](auto alternative) -> std::ostream& {
         if constexpr (std::is_same_v<
                           absl::remove_cvref_t<decltype(alternative)>,
                           absl::monostate>) {
-          // In optimized builds, we do nothing. In debug builds we cannot
-          // reach here.
-          return out;
+          return out << "default ctor ValueView";
         } else {
           return out << alternative;
         }

common/value.h

@@ -84,6 +84,9 @@ class Value final {
       : variant_((other.AssertIsValid(), other.variant_)) {}
 
   Value& operator=(const Value& other) {
+    if (this == std::addressof(other)) {
+      return *this;
+    }
     other.AssertIsValid();
     ABSL_DCHECK(this != std::addressof(other))
         << "Value should not be copied to itself";

common/value_test.cc

@@ -44,13 +44,14 @@ TEST(Value, GetTypeName) {
   EXPECT_DEBUG_DEATH(static_cast<void>(moved_from_value.GetTypeName()), _);
 }
 
-TEST(Value, DebugStringDebugDeath) {
+TEST(Value, DebugStringUinitializedValue) {
   Value moved_from_value = BoolValue(true);
   Value value = std::move(moved_from_value);
   IS_INITIALIZED(moved_from_value);
   static_cast<void>(value);
   std::ostringstream out;
-  EXPECT_DEBUG_DEATH(static_cast<void>(out << moved_from_value), _);
+  out << moved_from_value;
+  EXPECT_EQ(out.str(), "default ctor Value");
 }
 
 TEST(Value, NativeValueIdDebugDeath) {

common/value_testing.cc

@@ -28,9 +28,7 @@
 
 namespace cel {
 
-void PrintTo(const Value& value, std::ostream* os) {
-  *os << value.DebugString() << "\n";
-}
+void PrintTo(const Value& value, std::ostream* os) { *os << value << "\n"; }
 
 namespace test {
 namespace {

conformance/BUILD

@@ -167,6 +167,7 @@ cc_binary(
     for args in [
         [],
         ["--opt"],
+        ["--recursive"],
     ]
 ]
 
@@ -236,6 +237,11 @@ cc_binary(
             "--arena",
             "--opt",
         ],
+        [
+            "--modern",
+            "--arena",
+            "--recursive",
+        ],
     ]
 ]
 

conformance/server.cc

@@ -78,6 +78,9 @@ ABSL_FLAG(
 ABSL_FLAG(bool, arena, false,
           "Use arena memory manager (default: global heap ref-counted). Only "
           "affects the modern implementation");
+ABSL_FLAG(bool, recursive, false,
+          "Enable recursive plans. Depth limited to slightly more than the "
+          "default nesting limit.");
 
 namespace google::api::expr::runtime {
 
@@ -163,6 +166,10 @@ class LegacyConformanceServiceImpl : public ConformanceServiceInterface {
       options.constant_arena = constant_arena;
     }
 
+    if (absl::GetFlag(FLAGS_recursive)) {
+      options.max_recursion_depth = 48;
+    }
+
     std::unique_ptr<CelExpressionBuilder> builder =
         CreateCelExpressionBuilder(options);
     auto type_registry = builder->GetTypeRegistry();
@@ -284,6 +291,9 @@ class ModernConformanceServiceImpl : public ConformanceServiceInterface {
     options.enable_timestamp_duration_overflow_errors = true;
     options.enable_heterogeneous_equality = true;
     options.enable_empty_wrapper_null_unboxing = true;
+    if (absl::GetFlag(FLAGS_recursive)) {
+      options.max_recursion_depth = 48;
+    }
 
     return absl::WrapUnique(
         new ModernConformanceServiceImpl(options, use_arena, optimize));

eval/compiler/BUILD

@@ -28,8 +28,12 @@ cc_library(
         "//base:ast",
         "//base/ast_internal:ast_impl",
         "//base/ast_internal:expr",
+        "//common:native_type",
         "//common:value",
+        "//eval/eval:direct_expression_step",
         "//eval/eval:evaluator_core",
+        "//eval/eval:trace_step",
+        "//internal:casts",
         "//runtime:runtime_options",
         "//runtime/internal:issue_collector",
         "@com_google_absl//absl/algorithm:container",
@@ -40,6 +44,7 @@ cc_library(
         "@com_google_absl//absl/memory",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
+        "@com_google_absl//absl/types:optional",
         "@com_google_absl//absl/types:variant",
     ],
 )
@@ -51,10 +56,14 @@ cc_test(
         ":flat_expr_builder_extensions",
         ":resolver",
         "//base/ast_internal:expr",
+        "//common:casting",
         "//common:memory",
+        "//common:native_type",
         "//common:value",
         "//eval/eval:const_value_step",
+        "//eval/eval:direct_expression_step",
         "//eval/eval:evaluator_core",
+        "//eval/eval:function_step",
         "//internal:status_macros",
         "//internal:testing",
         "//runtime:function_registry",
@@ -83,13 +92,16 @@ cc_library(
         "//base/ast_internal:ast_impl",
         "//base/ast_internal:expr",
         "//common:memory",
+        "//common:type",
         "//common:value",
+        "//eval/eval:attribute_trail",
         "//eval/eval:comprehension_step",
         "//eval/eval:const_value_step",
         "//eval/eval:container_access_step",
         "//eval/eval:create_list_step",
         "//eval/eval:create_map_step",
         "//eval/eval:create_struct_step",
+        "//eval/eval:direct_expression_step",
         "//eval/eval:evaluator_core",
         "//eval/eval:function_step",
         "//eval/eval:ident_step",
@@ -99,6 +111,7 @@ cc_library(
         "//eval/eval:select_step",
         "//eval/eval:shadowable_value_step",
         "//eval/eval:ternary_step",
+        "//eval/eval:trace_step",
         "//eval/public:ast_traverse_native",
         "//eval/public:ast_visitor_native",
         "//eval/public:cel_type_registry",
@@ -108,6 +121,7 @@ cc_library(
         "//runtime:runtime_issue",
         "//runtime:runtime_options",
         "//runtime:type_registry",
+        "//runtime/internal:convert_constant",
         "//runtime/internal:issue_collector",
         "@com_google_absl//absl/algorithm:container",
         "@com_google_absl//absl/base:core_headers",
@@ -119,6 +133,7 @@ cc_library(
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
         "@com_google_absl//absl/strings",
+        "@com_google_absl//absl/types:optional",
         "@com_google_absl//absl/types:span",
         "@com_google_absl//absl/types:variant",
     ],
@@ -201,6 +216,7 @@ cc_test(
         "//internal:status_macros",
         "//internal:testing",
         "//parser",
+        "//runtime",
         "//runtime:runtime_options",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/strings",
@@ -220,7 +236,9 @@ cc_library(
     deps = [
         ":flat_expr_builder",
         "//base:ast",
+        "//common:native_type",
         "//eval/eval:cel_expression_flat_impl",
+        "//eval/eval:direct_expression_step",
         "//eval/eval:evaluator_core",
         "//eval/public:cel_expression",
         "//extensions/protobuf:ast_converters",
@@ -243,15 +261,32 @@ cc_test(
     ],
     deps = [
         ":cel_expression_builder_flat_impl",
+        ":constant_folding",
+        ":regex_precompilation_optimization",
+        "//eval/eval:cel_expression_flat_impl",
         "//eval/public:activation",
         "//eval/public:builtin_func_registrar",
+        "//eval/public:cel_expression",
+        "//eval/public:cel_function",
+        "//eval/public:cel_value",
+        "//eval/public:portable_cel_function_adapter",
+        "//eval/public/containers:container_backed_map_impl",
+        "//eval/public/structs:cel_proto_wrapper",
+        "//eval/public/structs:protobuf_descriptor_type_provider",
         "//eval/public/testing:matchers",
+        "//extensions:bindings_ext",
+        "//extensions/protobuf:memory_manager",
+        "//internal:status_macros",
         "//internal:testing",
         "//parser",
+        "//parser:macro",
         "//runtime:runtime_options",
+        "@com_google_absl//absl/algorithm:container",
         "@com_google_absl//absl/status",
+        "@com_google_cel_spec//proto/test/v1/proto3:test_all_types_cc_proto",
         "@com_google_googleapis//google/api/expr/v1alpha1:checked_cc_proto",
         "@com_google_googleapis//google/api/expr/v1alpha1:syntax_cc_proto",
+        "@com_google_protobuf//:protobuf",
     ],
 )
 
@@ -274,6 +309,7 @@ cc_library(
         "//common:memory",
         "//common:value",
         "//eval/eval:const_value_step",
+        "//eval/eval:direct_expression_step",
         "//eval/eval:evaluator_core",
         "//internal:status_macros",
         "//runtime:activation",
@@ -462,14 +498,17 @@ cc_library(
         "//common:native_type",
         "//common:value",
         "//eval/eval:compiler_constant_step",
+        "//eval/eval:direct_expression_step",
         "//eval/eval:evaluator_core",
         "//eval/eval:regex_match_step",
         "//internal:casts",
         "//internal:status_macros",
+        "@com_google_absl//absl/base:nullability",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/status",
         "@com_google_absl//absl/strings",
         "@com_google_absl//absl/types:optional",
+        "@com_googlesource_code_re2//:re2",
     ],
 )
 
@@ -485,16 +524,18 @@ cc_test(
         "//base/ast_internal:ast_impl",
         "//base/ast_internal:expr",
         "//common:memory",
-        "//common:type",
         "//common:value",
-        "//eval/eval:cel_expression_flat_impl",
         "//eval/eval:evaluator_core",
+        "//eval/public:activation",
         "//eval/public:builtin_func_registrar",
+        "//eval/public:cel_expression",
         "//eval/public:cel_options",
+        "//eval/public:cel_value",
         "//internal:testing",
         "//parser",
         "//runtime:runtime_issue",
         "//runtime/internal:issue_collector",
+        "@com_google_absl//absl/status",
         "@com_google_googleapis//google/api/expr/v1alpha1:checked_cc_proto",
         "@com_google_googleapis//google/api/expr/v1alpha1:syntax_cc_proto",
         "@com_google_protobuf//:protobuf",

eval/compiler/cel_expression_builder_flat_impl.cc

@@ -27,7 +27,9 @@
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
 #include "base/ast.h"
+#include "common/native_type.h"
 #include "eval/eval/cel_expression_flat_impl.h"
+#include "eval/eval/direct_expression_step.h"
 #include "eval/eval/evaluator_core.h"
 #include "eval/public/cel_expression.h"
 #include "extensions/protobuf/ast_converters.h"
@@ -94,6 +96,14 @@ CelExpressionBuilderFlatImpl::CreateExpressionImpl(
       warnings->push_back(issue.ToStatus());
     }
   }
+  if (flat_expr_builder_.options().max_recursion_depth != 0 &&
+      impl.path().size() > 0 &&
+      // mainline expression is exactly one recursive step.
+      impl.subexpressions().front().size() == 1 &&
+      impl.path().front()->GetNativeTypeId() ==
+          cel::NativeTypeId::For<WrappedDirectStep>()) {
+    return CelExpressionRecursiveImpl::Create(std::move(impl));
+  }
 
   return std::make_unique<CelExpressionFlatImpl>(std::move(impl));
 }