[all-commits] [llvm/llvm-project] 3a7d43: [libc++] Require the exact assignment expression t...

[Yuxuan Chen via All-commits]

  Branch: refs/heads/users/yuxuanchen1997/libcxx-uninitialized-copy-assign
  Home:   https://github.com/llvm/llvm-project
  Commit: 3a7d4301e8b481f940c6eba85e5908bc9ce650c3
      https://github.com/llvm/llvm-project/commit/3a7d4301e8b481f940c6eba85e5908bc9ce650c3
  Author: Yuxuan Chen <ych at meta.com>
  Date:   2026-05-09 (Sat, 09 May 2026)

  Changed paths:
    M libcxx/include/__memory/uninitialized_algorithms.h
    A libcxx/test/libcxx/memory/uninitialized_allocator_copy_template_op_assign.pass.cpp

  Log Message:
  -----------
  [libc++] Require the exact assignment expression to be trivial in __uninitialized_allocator_copy_impl

__uninitialized_allocator_copy_impl has an optimization that replaces allocator_traits::construct with std::copy for raw pointer ranges when the element type is trivially copy constructible and trivially copy assignable.

The copy-assignment trait only checks whether assignment from const T& is trivial. That is weaker than the expression used by std::copy, which evaluates *out = *in. If overload resolution selects a different non-trivial assignment operator for that expression, std::copy can call that operator on uninitialized storage.

Check is_trivially_assignable<_Out&, _In&> instead. This matches the assignment expression used by std::copy, preserves the optimized path when that assignment is actually trivial, and falls back to placement construction otherwise.

Add a regression test with a type whose defaulted copy assignment is trivial but whose templated assignment operator is selected for non-const lvalue sources.

Tested with:
~/llvm-project/build-libcxx-fresh/bin/llvm-lit ~/llvm-project/libcxx/test/libcxx/memory/uninitialized_allocator_copy_template_op_assign.pass.cpp ~/llvm-project/libcxx/test/libcxx/memory/uninitialized_allocator_copy.pass.cpp -q



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications