Replace std::any with a custom solution. (#20251)

`std::any` leads to compiler errors in some versions of gcc during
constructibility trait checks.

Now that we can guarantee it, return by reference to avoid extra costs in
copies.

PiperOrigin-RevId: 723478744

Co-authored-by: Protobuf Team Bot <protobuf-github-bot@google.com>

src/google/protobuf/descriptor.h

@@ -31,7 +31,6 @@
 #ifndef GOOGLE_PROTOBUF_DESCRIPTOR_H__
 #define GOOGLE_PROTOBUF_DESCRIPTOR_H__
 
-#include <any>
 #include <atomic>
 #include <cstdint>
 #include <iterator>
@@ -2455,11 +2454,20 @@ class PROTOBUF_EXPORT DescriptorPool {
   friend class google::protobuf::descriptor_unittest::ValidationErrorTest;
   friend class ::google::protobuf::compiler::CommandLineInterface;
   friend class TextFormat;
+
+  struct MemoBase {
+    virtual ~MemoBase() = default;
+  };
+  template <typename T>
+  struct MemoData : MemoBase {
+    T value;
+  };
+
   // Memoize a projection of a field.  This is used to cache the results of
   // calling a function on a field, used for expensive descriptor calculations.
   template <typename Func>
-  auto MemoizeProjection(const FieldDescriptor* field, Func func) const {
-    using ResultT = decltype(func(field));
+  const auto& MemoizeProjection(const FieldDescriptor* field, Func func) const {
+    using ResultT = std::decay_t<decltype(func(field))>;
     ABSL_DCHECK(field->file()->pool() == this);
     static_assert(std::is_empty_v<Func>);
     // This static bool is unique per-Func, so its address can be used as a key.
@@ -2469,15 +2477,21 @@ class PROTOBUF_EXPORT DescriptorPool {
       absl::ReaderMutexLock lock(&field_memo_table_mutex_);
       auto it = field_memo_table_.find(key);
       if (it != field_memo_table_.end()) {
-        return std::any_cast<ResultT>(it->second);
+        return internal::DownCast<const MemoData<ResultT>&>(*it->second).value;
       }
     }
-    ResultT result = func(field);
+    auto result = std::make_unique<MemoData<ResultT>>();
+    result->value = func(field);
     {
       absl::MutexLock lock(&field_memo_table_mutex_);
-      field_memo_table_[key] = result;
+      auto& res = field_memo_table_[key];
+      // Only initialize the first time. We don't want to invalidate old
+      // references.
+      if (res == nullptr) {
+        res = std::move(result);
+      }
+      return internal::DownCast<const MemoData<ResultT>&>(*res).value;
     }
-    return result;
   }
   // Return true if the given name is a sub-symbol of any non-package
   // descriptor that already exists in the descriptor pool.  (The full
@@ -2537,9 +2551,12 @@ class PROTOBUF_EXPORT DescriptorPool {
   Symbol NewPlaceholderWithMutexHeld(absl::string_view name,
                                      PlaceholderType placeholder_type) const;
 
+#ifndef SWIG
   mutable absl::Mutex field_memo_table_mutex_;
-  mutable absl::flat_hash_map<std::pair<const void*, const void*>, std::any>
+  mutable absl::flat_hash_map<std::pair<const void*, const void*>,
+                              std::unique_ptr<MemoBase>>
       field_memo_table_ ABSL_GUARDED_BY(field_memo_table_mutex_);
+#endif  // SWIG
 
   // If fallback_database_ is nullptr, this is nullptr.  Otherwise, this is a
   // mutex which must be locked while accessing tables_.

src/google/protobuf/descriptor_unittest.cc

@@ -27,6 +27,7 @@
 #include <string>
 #include <thread>  // NOLINT
 #include <tuple>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -11819,8 +11820,8 @@ TEST_F(DescriptorPoolFeaturesTest, ResolvesFeaturesFor) {
 class DescriptorPoolMemoizationTest : public ::testing::Test {
  protected:
   template <typename Func>
-  auto MemoizeProjection(const DescriptorPool* pool,
-                         const FieldDescriptor* field, Func func) {
+  const auto& MemoizeProjection(const DescriptorPool* pool,
+                                const FieldDescriptor* field, Func func) {
     return pool->MemoizeProjection(field, func);
   };
 };
@@ -11834,15 +11835,20 @@ TEST_F(DescriptorPoolMemoizationTest, MemoizeProjectionBasic) {
   proto2_unittest::TestAllTypes message;
   const Descriptor* descriptor = message.GetDescriptor();
 
-  auto name = DescriptorPoolMemoizationTest::MemoizeProjection(
+  const auto& name = DescriptorPoolMemoizationTest::MemoizeProjection(
       descriptor->file()->pool(), descriptor->field(0), name_lambda);
-  auto dupe_name = DescriptorPoolMemoizationTest::MemoizeProjection(
+  const auto& dupe_name = DescriptorPoolMemoizationTest::MemoizeProjection(
       descriptor->file()->pool(), descriptor->field(0), name_lambda);
 
   ASSERT_EQ(counter, 1);
   ASSERT_EQ(name, "proto2_unittest.TestAllTypes.optional_int32");
   ASSERT_EQ(dupe_name, "proto2_unittest.TestAllTypes.optional_int32");
 
+  // Check that they are references aliasing the same object.
+  EXPECT_TRUE(
+      (std::is_same_v<decltype(name), const decltype(descriptor->name()) &>));
+  EXPECT_EQ(&name, &dupe_name);
+
   auto other_name = DescriptorPoolMemoizationTest::MemoizeProjection(
       descriptor->file()->pool(), descriptor->field(1), name_lambda);