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

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,157 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 HasEmpty = requires(R r) {
+  std::ranges::empty(r);
+  { r.empty() } -> std::same_as<bool>;
+};
+
+constexpr void test_empty_iota_sfinae() {
+  std::vector<int> ev;
+
+  // Both parameters are non-const
+  {
+    auto iv = std::views::iota(std::ranges::begin(ev), std::ranges::end(ev));
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  // Left parameter is const
+  {
+    auto iv = std::views::iota(std::ranges::begin(std::as_const(ev)), std::ranges::end(ev));
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  // Right parameter is const
+  {
+    auto iv = std::views::iota(std::ranges::begin(ev), std::ranges::end(std::as_const(ev)));
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  // Both parameters are const
+  {
+    auto iv = std::views::iota(std::ranges::begin(std::as_const(ev)), std::ranges::end(std::as_const(ev)));
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+}
+
+constexpr void test_nonempty_iota_sfinae() {
+  // Default ctr
+  {
+    std::ranges::iota_view<Int42<DefaultTo42>> iv;
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  // Value pass
+  {
+    std::ranges::iota_view<SomeInt> iv(SomeInt(94));
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+
+  {
+    std::vector<char> v;
+    auto it = std::back_inserter(v);
+    auto iv = std::views::iota(it);
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+  {
+    std::vector<char> v{'b', 'a', 'b', 'a', 'z', 'm', 't'};
+    auto it = std::back_inserter(v);
+    auto iv = std::views::iota(it);
+
+    static_assert(HasEmpty<decltype(iv)>);
+  }
+}
+
+constexpr void test_empty_iota() {
+  std::vector<int> ev;
+
+  // Both parameters are non-const
+  {
+    auto iv = std::views::iota(std::ranges::begin(ev), std::ranges::end(ev));
+
+    assert(iv.empty());
+  }
+  // Left parameter is const
+  {
+    auto iv = std::views::iota(std::ranges::begin(std::as_const(ev)), std::ranges::end(ev));
+
+    assert(iv.empty());
+  }
+  // Right parameter is const
+  {
+    auto iv = std::views::iota(std::ranges::begin(ev), std::ranges::end(std::as_const(ev)));
+
+    assert(iv.empty());
+  }
+  // Both parameters are const
+  {
+    auto iv = std::views::iota(std::ranges::begin(std::as_const(ev)), std::ranges::end(std::as_const(ev)));
+
+    assert(iv.empty());
+  }
----------------
ldionne wrote:

I don't really understand the purpose of these tests. We're interested in checking that `empty()` can be called on a `const iota_view`, not that we can call it on a mutable `iota_view` constructed from const iterators?

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