[libcxx-commits] [libcxx] [libc++] Require the exact assignment expression to be trivial in `__uninitialized_allocator_copy_impl` (PR #196648)

[Yuxuan Chen via libcxx-commits]

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@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <new>
+#include <type_traits>
+#include <vector>
+
+class Optional {
+public:
+  Optional()                           = default;
+  Optional(const Optional&)            = default;
+  Optional& operator=(const Optional&) = default;
+  Optional(Optional&&) {}
+
+  template <class Arg>
+  Optional& operator=(Arg&& rhs) {
+    assert(value_ != Poison);
+    value_ = rhs.value_;
+    return *this;
+  }
+
+  bool hasValue() const { return value_ == HasValue; }
+
+private:
+  static constexpr std::uint32_t Poison   = 0xBEBEBEBE;
+  static constexpr std::uint32_t NoValue  = 0;
+  static constexpr std::uint32_t HasValue = 1;
+
+  std::uint32_t value_ = NoValue;
+};
+
+static_assert(std::is_trivially_copy_constructible<Optional>::value, "");
+static_assert(std::is_trivially_copy_assignable<Optional>::value, "");
+static_assert(!std::is_trivially_assignable<Optional&, Optional&>::value, "");
+
+template <class T>
+struct PoisoningAllocator {
+  using value_type     = T;
+  PoisoningAllocator() = default;
+  template <class U>
+  PoisoningAllocator(const PoisoningAllocator<U>&) noexcept {}
+  T* allocate(std::size_t n) {
+    void* p = ::operator new(n * sizeof(T));
+    std::memset(p, 0xBE, n * sizeof(T));
+    return static_cast<T*>(p);
+  }
+  void deallocate(T* p, std::size_t) noexcept { ::operator delete(p); }
+};
+
+template <class T, class U>
+bool operator==(const PoisoningAllocator<T>&, const PoisoningAllocator<U>&) noexcept {
+  return true;
+}
+template <class T, class U>
+bool operator!=(const PoisoningAllocator<T>&, const PoisoningAllocator<U>&) noexcept {
+  return false;
+}
+
+int main(int, char**) {
----------------
yuxuanchen1997 wrote:

I added the constexpr path, but after testing it against the old implementation I don’t think it actually covers this bug because `__uninitialized_allocator_copy_impl` already has a constant-evaluation branch [here](https://github.com/llvm/llvm-project/blob/45e5bfb456409d171db3a8f7fa8a553dc7bde0d8/libcxx/include/__memory/uninitialized_algorithms.h#L495). What we are fixing is specifically in the non-constexpr `std::copy` path. 

https://github.com/llvm/llvm-project/pull/196648