[libcxx-commits] [libcxx] [libc++][ranges] LWG4001: `iota_view` should provide `empty` (PR #79687)

[Konstantin Varlamov via libcxx-commits]

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@@ -0,0 +1,136 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// constexpr bool empty() const;
+
+#include <cassert>
+#include <concepts>
+#include <ranges>
+#include <utility>
+#include <vector>
+
+#include "types.h"
+
+template <typename R>
+concept HasFreeEmpty = requires(R r) { std::ranges::empty(r); };
+
+template <typename R>
+concept HasMemberEmpty = requires(R r) {
+  { r.empty() } -> std::same_as<bool>;
+};
+
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var-const wrote:

Nit: there is no case where we expect to have the member function but not the free function, so I think adding a third concept that tests both of these together would cut on some boilerplate (and more importantly, make it obvious that we never expect something like `static_assert(HasFreeEmpty<Foo>); static_assert(!HasMemberEmpty<Foo>);`).

https://github.com/llvm/llvm-project/pull/79687