Fix C++20 build: allocators + bundled LevelDB

C++20 broke two things in the prior bump:

1. std::allocator no longer exposes pointer/const_pointer/reference/
   const_reference member typedefs, and the 2-arg allocate(n, hint) was
   removed. Both secure_allocator and zero_after_free_allocator inherited
   these from std::allocator. Define the typedefs ourselves and switch
   the secure_allocator allocate() to the single-arg form.

2. Bundled src/leveldb uses `std::memory_order::memory_order_relaxed`
   which was valid in C++17 but became a hard error in C++20 (memory_order
   is now a scoped enum class — the values are at namespace scope or
   memory_order::relaxed, not memory_order::memory_order_relaxed). LevelDB
   itself only needs C++11, so pin its targets to C++17 in BuildLevelDB.cmake
   instead of patching vendored code.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmake/BuildLevelDB.cmake

@@ -9,6 +9,20 @@ if(NOT TARGET leveldb_lib)
     add_subdirectory("${LEVELDB_SOURCE_DIR}" "${LEVELDB_BINARY_DIR}")
 endif()
 
+# Pin bundled LevelDB to C++17. It only needs C++11 (declared via its own
+# target_compile_features) but inherits CMAKE_CXX_STANDARD=20 from the
+# top-level project, where some of its atomic-enum syntax
+# (std::memory_order::memory_order_relaxed) becomes a hard error.
+foreach(_leveldb_target leveldb_lib leveldb_memenv)
+    if(TARGET ${_leveldb_target})
+        set_target_properties(${_leveldb_target} PROPERTIES
+            CXX_STANDARD 17
+            CXX_STANDARD_REQUIRED ON
+            CXX_EXTENSIONS OFF
+        )
+    endif()
+endforeach()
+
 if(NOT TARGET build_leveldb)
     add_custom_target(build_leveldb DEPENDS leveldb_lib leveldb_memenv)
 endif()

src/allocators.h

@@ -182,15 +182,17 @@ private:
 template<typename T>
 struct secure_allocator : public std::allocator<T>
 {
-    // MSVC8 default copy constructor is broken
+    // C++20 removed pointer/reference/etc. member typedefs from std::allocator
+    // and removed the 2-arg allocate(n, hint). Define what we still need
+    // directly instead of pulling from base.
     typedef std::allocator<T> base;
-    typedef typename base::size_type size_type;
-    typedef typename base::difference_type  difference_type;
-    typedef typename base::pointer pointer;
-    typedef typename base::const_pointer const_pointer;
-    typedef typename base::reference reference;
-    typedef typename base::const_reference const_reference;
-    typedef typename base::value_type value_type;
+    typedef T              value_type;
+    typedef T*             pointer;
+    typedef const T*       const_pointer;
+    typedef T&             reference;
+    typedef const T&       const_reference;
+    typedef std::size_t    size_type;
+    typedef std::ptrdiff_t difference_type;
     secure_allocator() throw() {}
     secure_allocator(const secure_allocator& a) throw() : base(a) {}
     template <typename U>
@@ -199,10 +201,9 @@ struct secure_allocator : public std::allocator<T>
     template<typename _Other> struct rebind
     { typedef secure_allocator<_Other> other; };
 
-    T* allocate(std::size_t n, const void *hint = 0)
+    T* allocate(std::size_t n)
     {
-        T *p;
-        p = std::allocator<T>::allocate(n, hint);
+        T* p = std::allocator<T>::allocate(n);
         if (p != NULL)
             LockedPageManager::instance.LockRange(p, sizeof(T) * n);
         return p;
@@ -226,15 +227,16 @@ struct secure_allocator : public std::allocator<T>
 template<typename T>
 struct zero_after_free_allocator : public std::allocator<T>
 {
-    // MSVC8 default copy constructor is broken
+    // C++20 removed pointer/reference/etc. member typedefs from std::allocator.
+    // Define what we still need directly instead of pulling from base.
     typedef std::allocator<T> base;
-    typedef typename base::size_type size_type;
-    typedef typename base::difference_type  difference_type;
-    typedef typename base::pointer pointer;
-    typedef typename base::const_pointer const_pointer;
-    typedef typename base::reference reference;
-    typedef typename base::const_reference const_reference;
-    typedef typename base::value_type value_type;
+    typedef T              value_type;
+    typedef T*             pointer;
+    typedef const T*       const_pointer;
+    typedef T&             reference;
+    typedef const T&       const_reference;
+    typedef std::size_t    size_type;
+    typedef std::ptrdiff_t difference_type;
     zero_after_free_allocator() throw() {}
     zero_after_free_allocator(const zero_after_free_allocator& a) throw() : base(a) {}
     template <typename U>