[libcxx-commits] [libcxx] [libc++] Partially implement P2846R6: `reserve_hint` (PR #206385)

[Hristo Hristov via libcxx-commits]

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+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// constexpr auto reserve_hint()
+//     requires approximately_sized_range<V>;
+// constexpr auto reserve_hint() const
+//     requires approximately_sized_range<const V>;
+
+#include <cassert>
+#include <ranges>
+
+#include "test_iterators.h"
+
+// forward_iterator + sentinel end so that ranges::size doesn't apply,
+// making these ranges approximately_sized only via their member reserve_hint().
+
+struct ConstReserveHintView : std::ranges::view_base {
+  bool* hint_called;
+  constexpr auto begin() const { return forward_iterator<int*>(nullptr); }
+  constexpr auto end() const { return sentinel_wrapper<forward_iterator<int*>>(forward_iterator<int*>(nullptr)); }
+
+  constexpr unsigned int reserve_hint() const {
+    *hint_called = true;
+    return 3;
+  }
+};
+
+struct NonConstReserveHintView : std::ranges::view_base {
+  bool* hint_called;
+  constexpr auto begin() const { return forward_iterator<int*>(nullptr); }
+  constexpr auto end() const { return sentinel_wrapper<forward_iterator<int*>>(forward_iterator<int*>(nullptr)); }
+
+  constexpr unsigned int reserve_hint() {
+    *hint_called = true;
+    return 5;
+  }
+};
+
+struct NoReserveHintView : std::ranges::view_base {
+  constexpr auto begin() const { return forward_iterator<int*>(nullptr); }
+  constexpr auto end() const { return sentinel_wrapper<forward_iterator<int*>>(forward_iterator<int*>(nullptr)); }
+};
+
+template <class T>
+concept HasReserveHint = requires(T v) { v.reserve_hint(); };
+
+static_assert(!std::ranges::sized_range<std::ranges::as_rvalue_view<ConstReserveHintView>>);
+static_assert(!std::ranges::sized_range<std::ranges::as_rvalue_view<NonConstReserveHintView>>);
+static_assert(!std::ranges::sized_range<std::ranges::as_rvalue_view<NoReserveHintView>>);
+
+static_assert(HasReserveHint<std::ranges::as_rvalue_view<ConstReserveHintView>>);
+static_assert(HasReserveHint<const std::ranges::as_rvalue_view<ConstReserveHintView>>);
+static_assert(HasReserveHint<std::ranges::as_rvalue_view<NonConstReserveHintView>>);
+static_assert(!HasReserveHint<const std::ranges::as_rvalue_view<NonConstReserveHintView>>);
+static_assert(!HasReserveHint<std::ranges::as_rvalue_view<NoReserveHintView>>);
+static_assert(!HasReserveHint<const std::ranges::as_rvalue_view<NoReserveHintView>>);
+
+constexpr bool test() {
+  {
+    bool hint_called = false;
+    std::ranges::as_rvalue_view view(ConstReserveHintView{{}, &hint_called});
+    assert(view.reserve_hint() == 3);
+    assert(hint_called);
----------------
Zingam wrote:

```suggestion
    std::ranges::as_rvalue_view view(ConstReserveHintView{});
    assert(view.reserve_hint() == 3);
    assert(view.hint_called);
```
Is there any reason this simpler version won't work?

https://github.com/llvm/llvm-project/pull/206385