[steve-downey]

Every clang job built against libstdc++ fails beman::expected type traits match std::expected on main, for clang 19 through 22 and every C++ mode. The gcc jobs and the clang jobs on libc++ pass. The failing check is is_trivially_copyable for expected<int, MoveOnly> and expected<void, MoveOnly> (and the same for NoexceptMoveOnly).

Cause

Two things combine.

• libstdc++ 16 made std::expected<int, MoveOnly> trivially copyable. The test checks "beman is at least as trivial as std", which is an implication, so until the CI images moved to libstdc++ 16 there was nothing to check.

• Clang gets beman::expected<int, MoveOnly> wrong. For a move-only E, neither the trivial nor the non-trivial copy assignment is satisfied. [class.prop] counts only eligible special members, so the class is trivially copyable, and gcc agrees. Clang (17 through at least 24) treats a class whose copy-assignment candidates all have unsatisfied constraints as not trivially copyable:

  template <bool B> struct C {
      C(const C&) = delete;
      C(C&&) = default;
      C& operator=(const C&) requires B = default;
      C& operator=(C&&) = default;
  };
  static_assert(__is_trivially_copyable(C<false>));  // gcc: passes; clang: fails

Fix

The primary template, expected<void, E> and expected<T&, E> each get a deleted copy assignment that is satisfied exactly when neither existing path is. A deleted candidate is not eligible either, so the triviality it restores is the one the standard gives, and is_copy_assignable is unchanged. Move assignment is left alone: a satisfied deleted move assignment would stop an rvalue falling back to copy assignment for an E with a deleted move.

The new overload is \omitted from the generated wording, and its rationale sits after the primary template rather than on the declaration, because a comment on an omitted member is still carried into the synopsis.

The equivalence test's skip for Immovable was covering the same clang bug, so it is removed. expected_trivial.test.cpp gains a direct check that expected<int|void|int&, MoveOnly> is trivially copyable and not copy-assignable; unlike the equivalence test, it also runs on libc++.

Toolchains

The local clang-N toolchains all use libc++, where the equivalence test is skipped, so this failure could not be reproduced with them. This adds clang-22-libstdc++ and clang-23-libstdc++ (make TOOLCHAIN=clang-22-libstdc++), which share cmake/clang-libstdc++-flags.cmake, plus a plain clang-23 toolchain.

Testing

• make TOOLCHAIN=clang-22-libstdc++: fails before the fix with the same four CI failures; passes after, 1179/1179.
• make TOOLCHAIN=clang-23-libstdc++ and make TOOLCHAIN=gcc-16: 1179/1179.

Wording

Adding a member to expected<void, E> makes specgen emit the void_ alternative into the synopsis, because of a tie in how it orders edits: steve-downey/specgen#147. The checked-in wording is not regenerated here.