[flang] [llvm] [clang-tools-extra] [libc] [libcxx] [clang] [compiler-rt] [libc++] Implement LWG3940: std::expected<void, E>::value() also needs E to be copy constructible (PR #71819)

[Mark de Wever via cfe-commits]

================
@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+// 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, c++11, c++14, c++17, c++20
+// UNSUPPORTED: no-rtti
+// UNSUPPORTED: no-exceptions
+
+// constexpr void value() const &;
+// Mandates: is_copy_constructible_v<E> is true.
+
+// constexpr void value() &&;
+// Mandates: is_copy_constructible_v<E> is true and is_move_constructible_v<E> is true.
+
+#include <expected>
+
+#include "MoveOnly.h"
+
+struct CopyOnly {
+  CopyOnly()                = default;
+  CopyOnly(const CopyOnly&) = default;
+  CopyOnly(CopyOnly&&)      = delete;
+};
+
+void test() {
+  // MoveOnly type as error_type
+  std::expected<void, MoveOnly> e(std::unexpect, 5);
+
+  e.value(); // expected-note {{in instantiation of member function 'std::expected<void, MoveOnly>::value' requested here}}
+  // expected-error@*:* {{static assertion failed due to requirement 'is_copy_constructible_v<MoveOnly>'}}
+
+  std::move(e)
+      .value(); // expected-note {{in instantiation of member function 'std::expected<void, MoveOnly>::value' requested here}}
+  // expected-error@*:* {{static assertion failed due to requirement 'is_copy_constructible_v<MoveOnly>': error_type has to be both copy constructible and move constructible}}
+
+  // CopyOnly type as error_type
+  std::expected<void, CopyOnly> e2(std::unexpect);
+
+  e2.value();
+
+  std::move(e2)
+      .value(); // expected-note {{in instantiation of member function 'std::expected<void, CopyOnly>::value' requested here}}
+  // expected-error@*:* {{static assertion failed due to requirement 'is_move_constructible_v<CopyOnly>': error_type has to be both copy constructible and move constructible}}
+
+  // expected-error@*:* {{call to deleted constructor of 'MoveOnly'}}
+  // expected-error@*:* {{call to deleted constructor of 'CopyOnly'}}
+  // expected-error@*:* {{call to deleted constructor of 'CopyOnly'}}
----------------
mordante wrote:

Can you move these expected-error's to the places where they are generated. We need the `@*:*` since they are in the headers, but for maintainability it's good to have them where they are generated. 

https://github.com/llvm/llvm-project/pull/71819