[libcxx-commits] [libcxx] WIP [libc++][ranges] Applied [[nodiscard]] to `transform_view` (PR #204014)

[Hristo Hristov via libcxx-commits]

================
@@ -0,0 +1,138 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// Test the libc++ extension that std::ranges::transform_view and std::views::transform are marked as [[nodiscard]].
+
+#include <ranges>
+#include <functional>
+
+struct TestView : std::ranges::view_interface<TestView> {
+  int* begin();
+  char* begin() const;
+  const int* end();
+  const char* end() const;
+};
+
+void test() {
+  int range[] = {1, 2, 3};
+  auto f      = [](int i) { return i; };
+
+  auto identity_view     = TestView{} | std::views::transform(std::identity{});
+  auto transformed_range = range | std::views::transform(f);
+
+  const auto const_identity_view     = TestView{} | std::views::transform(std::identity{});
+  const auto const_transformed_range = range | std::views::transform(f);
+
+  auto it             = identity_view.begin();
+  const auto const_it = identity_view.begin();
+
+  auto transformed_range_sent             = transformed_range.end();
+  auto identity_view_sent                 = identity_view.end();
+  const auto const_transformed_range_sent = transformed_range.end();
+  const auto const_identity_view_sent     = identity_view.end();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  const_it.base();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  it.base();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::transform(identity_view, fn).base();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  transformed_range.begin();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  const_transformed_range.begin();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  transformed_range.end();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  identity_view.end();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  const_transformed_range.end();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  const_identity_view.end();
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::transform(f);
+
+  // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::views::transform(range, f);
+
+
+  const_it[0];
+
+  {
+    // ===== Non-const views =====
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    it + 1;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    1 + it;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    it - 1;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    it - it;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    *it;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    it[0];
+  }
+
+  {
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    const_it + 1;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    1 + const_it;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    const_it - 1;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    const_it - const_it;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    *const_it;
+
+    // expected-warning at +1 {{ignoring return value of function declared with 'nodiscard' attribute}}
+    const_it[0];
+  }
+
+  {
+    // ===== Non-const sentinels =====
----------------
Zingam wrote:

```suggestion
    // Non-const sentinels
```
The code base doesn't use this sort of "separators". Let's stick to what's the unwritten convention. Comments are like sentences.

Also it would be better to test const and non-const versions intermixed, e.g. group by function/operator name not by consteness. Most test were written like that and I think it's better to try to be consistent. You can also use `std::move()`, `std::as_const()` to test the move, const versions, which we use quite alot, for example see: https://github.com/llvm/llvm-project/blob/main/libcxx/test/libcxx/ranges/range.adaptors/range.as_rvalue/nodiscard.verify.cpp
or
https://github.com/llvm/llvm-project/blame/main/libcxx/test/libcxx/ranges/range.adaptors/range.concat/nodiscard.verify.cpp

https://github.com/llvm/llvm-project/pull/204014