Merge #47: Fix C++23 compilation errors in leveldb

183e79a Fix speculatively some "placement new" issues in leveldb (leveldb Team)
82c3104 Fix C++23 compilation errors in leveldb (leveldb Team)

Pull request description:

  Cherry-picks two commits from upstream (google@302786e, google@e829478), which remove the usage of `std::aligned_storage/std::aligned_union`.

  Note the first cherry-pick is not clean, because due to Google tooling issues, it accidently contained a revert of the prior two commits. See google#1249 for more details, as well as https://issues.chromium.org/issues/388068052.

  Closes #43.

ACKs for top commit:
  dergoegge:
    utACK 183e79a
  sipa:
    ACK 183e79a. I don't think the second commit actually changes anything.

Tree-SHA512: f2a5981451e34a5c6918cf070509020b1d6934591aad0307a44683ed0b9f8d75d7dcc54967366a4aa46c4ecc4b5108b43514cf1045769f1b68eab5359401889b

Author: fanquake

util/env_posix.cc

@@ -854,9 +854,13 @@ class SingletonEnv {
 #endif  // !defined(NDEBUG)
     static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                   "env_storage_ will not fit the Env");
-    static_assert(alignof(decltype(env_storage_)) >= alignof(EnvType),
+    static_assert(std::is_standard_layout_v<SingletonEnv<EnvType>>);
+    static_assert(
+        offsetof(SingletonEnv<EnvType>, env_storage_) % alignof(EnvType) == 0,
+        "env_storage_ does not meet the Env's alignment needs");
+    static_assert(alignof(SingletonEnv<EnvType>) % alignof(EnvType) == 0,
                   "env_storage_ does not meet the Env's alignment needs");
-    new (&env_storage_) EnvType();
+    new (env_storage_) EnvType();
   }
   ~SingletonEnv() = default;
 
@@ -872,8 +876,7 @@ class SingletonEnv {
   }
 
  private:
-  typename std::aligned_storage<sizeof(EnvType), alignof(EnvType)>::type
-      env_storage_;
+  alignas(EnvType) char env_storage_[sizeof(EnvType)];
 #if !defined(NDEBUG)
   static std::atomic<bool> env_initialized_;
 #endif  // !defined(NDEBUG)

util/env_windows.cc

@@ -802,9 +802,13 @@ class SingletonEnv {
 #endif  // !defined(NDEBUG)
     static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                   "env_storage_ will not fit the Env");
-    static_assert(alignof(decltype(env_storage_)) >= alignof(EnvType),
+    static_assert(std::is_standard_layout_v<SingletonEnv<EnvType>>);
+    static_assert(
+        offsetof(SingletonEnv<EnvType>, env_storage_) % alignof(EnvType) == 0,
+        "env_storage_ does not meet the Env's alignment needs");
+    static_assert(alignof(SingletonEnv<EnvType>) % alignof(EnvType) == 0,
                   "env_storage_ does not meet the Env's alignment needs");
-    new (&env_storage_) EnvType();
+    new (env_storage_) EnvType();
   }
   ~SingletonEnv() = default;
 
@@ -820,8 +824,7 @@ class SingletonEnv {
   }
 
  private:
-  typename std::aligned_storage<sizeof(EnvType), alignof(EnvType)>::type
-      env_storage_;
+  alignas(EnvType) char env_storage_[sizeof(EnvType)];
 #if !defined(NDEBUG)
   static std::atomic<bool> env_initialized_;
 #endif  // !defined(NDEBUG)

util/no_destructor.h

@@ -5,6 +5,7 @@
 #ifndef STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
 #define STORAGE_LEVELDB_UTIL_NO_DESTRUCTOR_H_
 
+#include <cstddef>
 #include <type_traits>
 #include <utility>
 
@@ -20,10 +21,14 @@ class NoDestructor {
   explicit NoDestructor(ConstructorArgTypes&&... constructor_args) {
     static_assert(sizeof(instance_storage_) >= sizeof(InstanceType),
                   "instance_storage_ is not large enough to hold the instance");
+    static_assert(std::is_standard_layout_v<NoDestructor<InstanceType>>);
     static_assert(
-        alignof(decltype(instance_storage_)) >= alignof(InstanceType),
+        offsetof(NoDestructor, instance_storage_) % alignof(InstanceType) == 0,
         "instance_storage_ does not meet the instance's alignment requirement");
-    new (&instance_storage_)
+    static_assert(
+        alignof(NoDestructor<InstanceType>) % alignof(InstanceType) == 0,
+        "instance_storage_ does not meet the instance's alignment requirement");
+    new (instance_storage_)
         InstanceType(std::forward<ConstructorArgTypes>(constructor_args)...);
   }
 
@@ -37,8 +42,7 @@ class NoDestructor {
   }
 
  private:
-  typename std::aligned_storage<sizeof(InstanceType),
-                                alignof(InstanceType)>::type instance_storage_;
+  alignas(InstanceType) char instance_storage_[sizeof(InstanceType)];
 };
 
 }  // namespace leveldb