[llvm] [clang-tools-extra] [libcxx] [clang] [libc++][span] P2821R5: span.at() (PR #74994)

[Nikolas Klauser via cfe-commits]

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@@ -0,0 +1,136 @@
+//===----------------------------------------------------------------------===//
+//
+// 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, c++20, c++23
+
+// <span>
+
+// constexpr reference at(size_type idx) const; // since C++26
+
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <span>
+#include <stdexcept>
+#include <utility>
+#include <vector>
+
+#include "test_macros.h"
+
+constexpr void testSpanAt(auto span, int idx, int expectedValue) {
+  // non-const
+  {
+    std::same_as<typename decltype(span)::reference> decltype(auto) elem = span.at(idx);
+    assert(elem == expectedValue);
+  }
+
+  // const
+  {
+    std::same_as<typename decltype(span)::reference> decltype(auto) elem = std::as_const(span).at(idx);
+    assert(elem == expectedValue);
+  }
+}
+
+constexpr bool test() {
+  // With static extent
+  {
+    std::array arr{0, 1, 2, 3, 4, 5, 9084};
+    std::span arrSpan{arr};
+
+    assert(std::dynamic_extent != arrSpan.extent);
+
+    testSpanAt(arrSpan, 0, 0);
+    testSpanAt(arrSpan, 1, 1);
+    testSpanAt(arrSpan, 6, 9084);
+  }
+
+  // With dynamic extent
+  {
+    std::vector vec{0, 1, 2, 3, 4, 5, 9084};
+    std::span vecSpan{vec};
+
+    assert(std::dynamic_extent == vecSpan.extent);
+
+    testSpanAt(vecSpan, 0, 0);
+    testSpanAt(vecSpan, 1, 1);
+    testSpanAt(vecSpan, 6, 9084);
+  }
+
+  return true;
+}
+
+void test_exceptions() {
+#ifndef TEST_HAS_NO_EXCEPTIONS
+  // With static extent
+  {
+    std::array arr{1, 2, 3, 4};
+    const std::span arrSpan{arr};
+
+    try {
+      TEST_IGNORE_NODISCARD arrSpan.at(arr.size() + 1);
----------------
philnik777 wrote:

For what reason though? In which case is `TEST_IGNORE_NODISCARD` better to understand than `(void)`? I'm pretty sure I've pushed back against uses of this macro in the past and haven't gotten any good response for why one would ever use it. IMO this makes things just more confusing.

https://github.com/llvm/llvm-project/pull/74994