make get nodiscard, add nodiscard test, clean up tests

Author: Tsche

libcxx/include/__utility/integer_sequence.h

@@ -70,7 +70,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __for_each_index_sequence(index_sequence<_I
 #    endif // _LIBCPP_STD_VER >= 20
 
 #    if _LIBCPP_STD_VER >= 26
-// structured binding support for integer_sequence
+// [intseq.binding], structured binding support
 template <class _Tp, _Tp... _Indices>
 struct tuple_size<integer_sequence<_Tp, _Indices...>> : integral_constant<size_t, sizeof...(_Indices)> {};
 
@@ -87,7 +87,7 @@ struct tuple_element<_Ip, const integer_sequence<_Tp, _Indices...>> {
 };
 
 template <size_t _Ip, class _Tp, _Tp... _Indices>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp get(integer_sequence<_Tp, _Indices...>) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp get(integer_sequence<_Tp, _Indices...>) noexcept {
   static_assert(_Ip < sizeof...(_Indices), "Index out of bounds in std::get<> (std::integer_sequence)");
   return _Indices...[_Ip];
 }

libcxx/test/std/utilities/intseq/intseq.binding/nodiscard.verify.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <utility>
+
+// template<size_t I, class T, T... Values>
+//   [[nodiscard]] constexpr T get(integer_sequence<T, Values...>) noexcept;
+
+// check that get is marked [[nodiscard]]
+
+#include <utility>
+
+void f() {
+  std::index_sequence<1> seq;
+  get<0>(seq);    // expected-warning {{ignoring return value of function}}
+}

libcxx/test/std/utilities/intseq/intseq.binding/structured_binding.pass.cpp

@@ -20,35 +20,34 @@
 #include <cassert>
 #include <utility>
 
-void test_structured_bindings() {
+constexpr bool test_structured_bindings() {
   auto [elt0, elt1, elt2, elt3] = std::make_index_sequence<4>();
-  
+
   assert(elt0 == 0);
   assert(elt1 == 1);
   assert(elt2 == 2);
   assert(elt3 == 3);
-}
 
 #if __cpp_structured_bindings >= 202411L
-template <typename...>
-void test_p1061_structured_bindings() {
-  auto [... empty] = std::make_index_sequence<0>();
-  static_assert(sizeof...(empty) == 0);
-
-  auto [... size4] = std::make_index_sequence<4>();
-  static_assert(sizeof...(size4) == 4);
-
-  assert(size4...[0] == 0);
-  assert(size4...[1] == 1);
-  assert(size4...[2] == 2);
-  assert(size4...[3] == 3);
-}
+  []<typename...> {
+    auto [... empty] = std::make_index_sequence<0>();
+    static_assert(sizeof...(empty) == 0);
+
+    auto [... size4] = std::make_index_sequence<4>();
+    static_assert(sizeof...(size4) == 4);
+
+    assert(size4...[0] == 0);
+    assert(size4...[1] == 1);
+    assert(size4...[2] == 2);
+    assert(size4...[3] == 3);
+  }();
 #endif
 
+  return true;
+}
+
 int main(int, char**) {
   test_structured_bindings();
-#if __cpp_structured_bindings >= 202411L
-  test_p1061_structured_bindings();
-#endif
+  static_assert(test_structured_bindings());
   return 0;
 }

libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.compile.pass.cpp

@@ -25,7 +25,7 @@
 #include <type_traits>
 #include <utility>
 
-constexpr void test() {
+void test() {
   // std::tuple_size_v
   using empty = std::integer_sequence<int>;
   static_assert(std::tuple_size_v<empty> == 0);
@@ -47,14 +47,12 @@ constexpr void test() {
   static_assert(std::is_same_v<std::tuple_element_t<3, const size4>, int>);
 
   // std::get
-  constexpr static size4 seq4{};
-  static_assert(get<0>(seq4) == 9);
+  constexpr static size4 seq4;
+  constexpr std::same_as<int> decltype(auto) elt0 = get<0>(seq4);
+  static_assert(elt0 == 9);
   static_assert(get<1>(seq4) == 8);
   static_assert(get<2>(seq4) == 7);
   static_assert(get<3>(seq4) == 2);
 
   static_assert(noexcept(get<0>(seq4)));
-
-  constexpr std::same_as<int> decltype(auto) r = get<0>(seq4);
-  static_assert(r == 9);
 }

libcxx/test/std/utilities/intseq/intseq.binding/tuple_interface.verify.cpp

@@ -27,7 +27,7 @@ void f() {
   // expected-error-re@*:* {{static assertion failed{{.*}}Index out of bounds in std::tuple_element<> (const std::integer_sequence)}}
   using test2 = std::tuple_element_t<0, const std::integer_sequence<int>>;
 
-  auto empty = std::integer_sequence<int>();
+  std::integer_sequence<int> empty;
   // expected-error-re@*:* {{static assertion failed{{.*}}Index out of bounds in std::get<> (std::integer_sequence)}}
   // expected-error-re@*:* {{invalid index 0 for pack {{.*}} of size 0}}
   (void)std::get<0>(empty);