[libcxx-commits] [libcxx] [libc++][ranges] Implement P1899 `ranges::stride_view`. (PR #65200)

[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++23
+
+// constexpr auto begin() requires(!__simple_view<_View>)
+// constexpr auto begin() const requires range<const _View>
+
+// Note: Checks here are augmented by checks in
+// iterator/ctor.copy.pass.cpp.
+
+#include <concepts>
+#include <ranges>
+
+#include "types.h"
+
+template <class T>
+concept HasConstBegin = requires(const T& ct) { ct.begin(); };
+
+template <class T>
+concept HasBegin = requires(T& t) { t.begin(); };
+
+template <class T>
+concept HasConstAndNonConstBegin = requires(T& t, const T& ct) {
+  // The return types for begin are different when this is const or not:
+  // the iterator's _Const non-type-template parameter is true in the former
+  // and false in the latter.
+  requires !std::same_as<decltype(t.begin()), decltype(ct.begin())>;
+};
+
+template <class T>
+// There is a begin but it's not const qualified => Only non-const qualified begin.
+concept HasOnlyNonConstBegin = HasBegin<T> && !HasConstBegin<T>;
+
+template <class T>
+// There is a const-qualified begin and there are not both const- and non-const qualified begin => Only const-qualified begin.
+concept HasOnlyConstBegin = HasConstBegin<T> && !HasConstAndNonConstBegin<T>;
+
+static_assert(HasOnlyNonConstBegin<std::ranges::stride_view<UnSimpleNoConstCommonView>>);
+static_assert(HasOnlyConstBegin<std::ranges::stride_view<SimpleCommonConstView>>);
+static_assert(HasConstAndNonConstBegin<std::ranges::stride_view<UnSimpleConstView>>);
+
+constexpr bool test() {
+  const auto unsimple_const_view = UnSimpleConstView();
+  const auto sv_unsimple_const   = std::ranges::stride_view(unsimple_const_view, 1);
+  static_assert(std::same_as<decltype(*sv_unsimple_const.begin()), double&>);
+
+  auto simple_const_view = SimpleCommonConstView();
+  auto sv_simple_const   = std::ranges::stride_view(simple_const_view, 1);
+  static_assert(std::same_as<decltype(*sv_simple_const.begin()), int&>);
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}
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H-G-Hristov wrote:

Please add an empty line at the end of the file as per convention.

https://github.com/llvm/llvm-project/pull/65200