[libcxx] [llvm] [libcxx] improves diagnostics for containers with bad value types (PR #106296)

[Louis Dionne via llvm-commits]

================
@@ -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
+//
+//===----------------------------------------------------------------------===//
+
+// Checks that we can only form unordered_multisets of object types.
+
+#include <unordered_set>
+#include "test_macros.h"
+
+std::unordered_multiset<const int> C1;
+// expected-error@*:*{{'std::unordered_multiset' cannot hold const types}}
+
+std::unordered_multiset<volatile int> C2;
+// expected-error@*:*{{'std::unordered_multiset' cannot hold volatile types}}
+
+std::unordered_multiset<int&> C3;
+std::unordered_multiset<int&&> C4;
+// expected-error@*:* 2 {{'std::unordered_multiset' cannot hold references}}
+
+std::unordered_multiset<int()> C5;
+std::unordered_multiset<int(int)> C6;
+std::unordered_multiset<int(int, int)> C7;
+// expected-error@*:* 3 {{'std::unordered_multiset' cannot hold functions}}
+
+std::unordered_multiset<void> C8;
+// expected-error@*:*{{'std::unordered_multiset' cannot hold 'void'}}
+
+std::unordered_multiset<int[]> C9;
+// expected-error@*:*{{'std::unordered_multiset' cannot hold C arrays of an unknown size}}
+
+// std::hash doesn't work with C arrays, so we need to test it with something else to ensure the
+// correct diagnostic is issued.
+template <class T>
+struct test_hash {
+  using argument_type = T;
+  using result_type   = std::size_t;
+
+  result_type operator()(T const&) const;
+};
+
+std::unordered_multiset<int[2], test_hash<int[2]> > C10;
+#if TEST_STD_VER < 20
+// expected-error@*:*{{'std::unordered_multiset' cannot hold C arrays before C++20}}
+#endif
+
+// Spurious errors
----------------
ldionne wrote:

I believe it would be better to use `// ADDITIONAL_COMPILE_FLAGS: -verify-ignore-unexpected=error` to avoid making this test too brittle to arbitrary Clang changes. This is something we've discussed quite a bit with Clang folks and we want to try not to couple our tests too much to Clang details, which causes disruption every time something starts failing for an unrelated reason.

https://github.com/llvm/llvm-project/pull/106296