[libc++][test] Improve test coverage for flat_set (lack of) SCARY iterators (#139649)

Missing from 5e94e26.

Author: halbi2

libcxx/test/libcxx/containers/container.adaptors/flat.set/scary.compile.pass.cpp

@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <flat_set>
+
+// class flat_set
+// class flat_multiset
+
+// Extension: SCARY/N2913 iterator compatibility between flat_set and flat_multiset
+// Test for the absence of this feature
+
+#include <flat_set>
+#include <type_traits>
+
+#include "test_macros.h"
+
+void test() {
+  typedef std::flat_set<int, int> M1;
+  typedef std::flat_multiset<int, int> M2;
+
+  static_assert(!std::is_convertible_v<M1::iterator, M2::iterator>);
+  static_assert(!std::is_convertible_v<M2::iterator, M1::iterator>);
+
+  static_assert(!std::is_convertible_v<M1::const_iterator, M2::const_iterator>);
+  static_assert(!std::is_convertible_v<M2::const_iterator, M1::const_iterator>);
+}