[libc++][ranges] Applied [[nodiscard]] to common_view

[H-G-Hristov]

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

• https://libcxx.llvm.org/CodingGuidelines.html
• https://wg21.link/ranges
• https://wg21.link/range.common

Towards #172124

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Hristo Hristov (H-G-Hristov)

Changes

[[nodiscard]] should be applied to functions where discarding the return value is most likely a correctness issue.

• https://libcxx.llvm.org/CodingGuidelines.html
• https://wg21.link/ranges
• https://wg21.link/range.common

Towards #172124

---

Full diff: https://github.com/llvm/llvm-project/pull/173176.diff

2 Files Affected:

• (modified) libcxx/include/__ranges/common_view.h (+7-7)
• (added) libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp (+61)

diff --git a/libcxx/include/__ranges/common_view.h b/libcxx/include/__ranges/common_view.h
index 133236dd1d78a..eec1045c8a758 100644
--- a/libcxx/include/__ranges/common_view.h
+++ b/libcxx/include/__ranges/common_view.h
@@ -56,16 +56,16 @@ class common_view : public view_interface<common_view<_View>> {
     return __base_;
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() {
     if constexpr (random_access_range<_View> && sized_range<_View>)
       return ranges::begin(__base_);
     else
       return common_iterator<iterator_t<_View>, sentinel_t<_View>>(ranges::begin(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
     requires range<const _View>
   {
     if constexpr (random_access_range<const _View> && sized_range<const _View>)
@@ -74,14 +74,14 @@ class common_view : public view_interface<common_view<_View>> {
       return common_iterator<iterator_t<const _View>, sentinel_t<const _View>>(ranges::begin(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() {
     if constexpr (random_access_range<_View> && sized_range<_View>)
       return ranges::begin(__base_) + ranges::size(__base_);
     else
       return common_iterator<iterator_t<_View>, sentinel_t<_View>>(ranges::end(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
     requires range<const _View>
   {
     if constexpr (random_access_range<const _View> && sized_range<const _View>)
@@ -90,13 +90,13 @@ class common_view : public view_interface<common_view<_View>> {
       return common_iterator<iterator_t<const _View>, sentinel_t<const _View>>(ranges::end(__base_));
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size()
     requires sized_range<_View>
   {
     return ranges::size(__base_);
   }
 
-  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
     requires sized_range<const _View>
   {
     return ranges::size(__base_);
diff --git a/libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp b/libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp
new file mode 100644
index 0000000000000..3542251b7d436
--- /dev/null
+++ b/libcxx/test/libcxx/ranges/range.adaptors/range.common.view/nodiscard.verify.cpp
@@ -0,0 +1,61 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++20
+
+// Check that functions are marked [[nodiscard]]
+
+#include <ranges>
+#include <utility>
+
+struct NonCommonView : std::ranges::view_base {
+  int* begin() const;
+  const int* end() const;
+
+  int* base();
+
+  int* begin();
+  const int* end();
+
+  constexpr std::size_t size() const { return 0; };
+};
+static_assert(!std::ranges::common_range<NonCommonView>);
+
+void test() {
+  NonCommonView range;
+
+  auto v = std::views::common(range);
+
+  // [range.common.view]
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::move(v).base();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.begin();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).begin();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.end();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).end();
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  v.size();
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::as_const(v).size();
+
+  // [range.common.overview]
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::common(v);
+
+  // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::common(range);
+}