container: count allocate_at_least in heap_vector test allocator (C++23) (#2665)

Summary:
HeapVectorTypes.GrowthPolicy checks CountingAllocator::nAllocations, but the allocator only overrode allocate(). Since C++23, std::vector grows via std::allocator_traits::allocate_at_least, which CountingAllocator inherited from std::allocator and which calls the base allocate(), bypassing the override. On Mac CI (Homebrew LLVM, libc++ in C++23 mode) this left nAllocations at 0 and failed the test; it passed locally under older libc++ / C++20, which still routes growth through allocate().

Override allocate_at_least() to also count, guarded on __cpp_lib_allocate_at_least so C++20 builds are unaffected. Reproduced and verified with -std=c++23 vs c++20.

Pull Request resolved: #2665

Reviewed By: ilvokhin

Differential Revision: D109797758

Pulled By: afrind

fbshipit-source-id: 4fc0be316c1f69fef1881253d9bfb6f6b88be0de

Author: afrind

folly/container/test/heap_vector_types_test.cpp

@@ -69,6 +69,15 @@ struct CountingAllocator : std::allocator<T> {
     nAllocations += 1;
     return std::allocator<T>::allocate(n);
   }
+#ifdef __cpp_lib_allocate_at_least
+  // Since C++23, std::vector allocates via allocator_traits::allocate_at_least,
+  // which std::allocator provides; without this override that inherited method
+  // would bypass the counting allocate() above and leave nAllocations at 0.
+  auto allocate_at_least(std::size_t n) {
+    nAllocations += 1;
+    return std::allocator<T>::allocate_at_least(n);
+  }
+#endif
   int nAllocations{0};
 
   template <typename U>