[libcxx-commits] [libcxx] [libc++] Implement `std::function_ref` (PR #186692)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,115 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// template<class T> function_ref& operator=(T) = delete;
+
+#include <cassert>
+#include <functional>
+#include <utility>
+#include <type_traits>
+
+#include "test_macros.h"
+
+// Constraints:
+// - T is not the same type as function_ref,
+// - is_pointer_v<T> is false, and
+// - T is not a specialization of constant_wrapper.
+
+// non const noexcept(false)
+static_assert(std::is_assignable_v<std::function_ref<void()>, std::function_ref<void()>>);
+static_assert(std::is_assignable_v<std::function_ref<void()>, std::function_ref<void() const>>);
+static_assert(std::is_assignable_v<std::function_ref<void()>, std::function_ref<void() noexcept>>);
+static_assert(std::is_assignable_v<std::function_ref<void()>, std::function_ref<void() const noexcept>>);
+
+static_assert(std::is_assignable_v<std::function_ref<void()>, void (*)()>);
+static_assert(!std::is_assignable_v<std::function_ref<void()>, void (*)(int)>);
+
+static_assert(std::is_assignable_v<std::function_ref<void()>, std::constant_wrapper<[] {}>>);
+static_assert(!std::is_assignable_v<std::function_ref<void()>, std::constant_wrapper<[](int) {}>>);
+
+// const noexcept(false)
+static_assert(!std::is_assignable_v<std::function_ref<void() const>, std::function_ref<void()>>);
+static_assert(std::is_assignable_v<std::function_ref<void() const>, std::function_ref<void() const>>);
+static_assert(!std::is_assignable_v<std::function_ref<void() const>, std::function_ref<void() noexcept>>);
+static_assert(std::is_assignable_v<std::function_ref<void() const>, std::function_ref<void() const noexcept>>);
+
+static_assert(std::is_assignable_v<std::function_ref<void() const>, void (*)()>);
+static_assert(!std::is_assignable_v<std::function_ref<void() const>, void (*)(int)>);
+
+static_assert(std::is_assignable_v<std::function_ref<void() const>, std::constant_wrapper<[] { return 42; }>>);
+static_assert(!std::is_assignable_v<std::function_ref<void() const>, std::constant_wrapper<[](int) { return 42; }>>);
+
+// non-const noexcept(true)
+static_assert(!std::is_assignable_v<std::function_ref<void() noexcept>, std::function_ref<void()>>);
+static_assert(!std::is_assignable_v<std::function_ref<void() noexcept>, std::function_ref<void() const>>);
+static_assert(std::is_assignable_v<std::function_ref<void() noexcept>, std::function_ref<void() noexcept>>);
+static_assert(std::is_assignable_v<std::function_ref<void() noexcept>, std::function_ref<void() const noexcept>>);
+
+static_assert(std::is_assignable_v<std::function_ref<void() noexcept>, void (*)() noexcept>);
+static_assert(!std::is_assignable_v<std::function_ref<void() noexcept>, void (*)(int) noexcept>);
+
+static_assert(std::is_assignable_v<std::function_ref<void() noexcept>, std::constant_wrapper<[] noexcept {} >>);
+static_assert(!std::is_assignable_v<std::function_ref<void() noexcept>, std::constant_wrapper<[](int) noexcept {}>>);
+
+// const noexcept(true)
+static_assert(!std::is_assignable_v<std::function_ref<void() const noexcept>, std::function_ref<void()>>);
+static_assert(!std::is_assignable_v<std::function_ref<void() const noexcept>, std::function_ref<void() const>>);
+static_assert(!std::is_assignable_v<std::function_ref<void() const noexcept>, std::function_ref<void() noexcept>>);
+static_assert(std::is_assignable_v<std::function_ref<void() const noexcept>, std::function_ref<void() const noexcept>>);
+
+static_assert(std::is_assignable_v<std::function_ref<void() const noexcept>, void (*)() noexcept>);
+static_assert(!std::is_assignable_v<std::function_ref<void() const noexcept>, void (*)(int) noexcept>);
+
+static_assert(std::is_assignable_v<std::function_ref<void() const noexcept>, std::constant_wrapper<[] noexcept {}>>);
+static_assert(
+    !std::is_assignable_v<std::function_ref<void() const noexcept>, std::constant_wrapper<[](int) noexcept {}>>);
+
+int forty_two() { return 42; }
+
+void test() {
+  {
+    std::function_ref<int()> f(std::cw<[] { return 41; }>);
+    f = std::function_ref<int()>(std::cw<[] { return 42; }>);
----------------
ldionne wrote:

We need to check the return type and the return value of this function.

```
std::same_as<function_ref<...>&> decltype(auto) result = (f = whatever);
assert(&result == &f);
```

https://github.com/llvm/llvm-project/pull/186692