[libc++] Decouple iterator_traits test from precise Clang diagnostics (#107478)

This makes the test more robust and prevents it from breaking when the
diagnostic changes subtly (e.g. under modules).

Author: ldionne

libcxx/test/std/iterators/iterator.primitives/iterator.traits/empty.compile.pass.cpp

@@ -0,0 +1,132 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <iterator>
+
+// struct iterator_traits
+// {
+// };
+
+#include <iterator>
+#include <type_traits>
+
+#include "test_macros.h"
+
+template <class...>
+using always_void = void;
+
+#define HAS_XXX(member)                                                                                                \
+  template <class T, class = void>                                                                                     \
+  struct has_##member : std::false_type {};                                                                            \
+  template <class T>                                                                                                   \
+  struct has_##member<T, always_void<typename T::member> > : std::true_type {}
+
+HAS_XXX(difference_type);
+HAS_XXX(value_type);
+HAS_XXX(pointer);
+HAS_XXX(reference);
+HAS_XXX(iterator_category);
+
+struct A {};
+struct NotAnIteratorEmpty {};
+
+struct NotAnIteratorNoDifference {
+  //     typedef int                       difference_type;
+  typedef A value_type;
+  typedef A* pointer;
+  typedef A& reference;
+  typedef std::forward_iterator_tag iterator_category;
+};
+
+struct NotAnIteratorNoValue {
+  typedef int difference_type;
+  //     typedef A                         value_type;
+  typedef A* pointer;
+  typedef A& reference;
+  typedef std::forward_iterator_tag iterator_category;
+};
+
+struct NotAnIteratorNoPointer {
+  typedef int difference_type;
+  typedef A value_type;
+  //     typedef A*                        pointer;
+  typedef A& reference;
+  typedef std::forward_iterator_tag iterator_category;
+};
+
+struct NotAnIteratorNoReference {
+  typedef int difference_type;
+  typedef A value_type;
+  typedef A* pointer;
+  //    typedef A&                        reference;
+  typedef std::forward_iterator_tag iterator_category;
+};
+
+struct NotAnIteratorNoCategory {
+  typedef int difference_type;
+  typedef A value_type;
+  typedef A* pointer;
+  typedef A& reference;
+  //     typedef std::forward_iterator_tag iterator_category;
+};
+
+void test() {
+  {
+    typedef std::iterator_traits<NotAnIteratorEmpty> T;
+    static_assert(!has_difference_type<T>::value, "");
+    static_assert(!has_value_type<T>::value, "");
+    static_assert(!has_pointer<T>::value, "");
+    static_assert(!has_reference<T>::value, "");
+    static_assert(!has_iterator_category<T>::value, "");
+  }
+
+  {
+    typedef std::iterator_traits<NotAnIteratorNoDifference> T;
+    static_assert(!has_difference_type<T>::value, "");
+    static_assert(!has_value_type<T>::value, "");
+    static_assert(!has_pointer<T>::value, "");
+    static_assert(!has_reference<T>::value, "");
+    static_assert(!has_iterator_category<T>::value, "");
+  }
+
+  {
+    typedef std::iterator_traits<NotAnIteratorNoValue> T;
+    static_assert(!has_difference_type<T>::value, "");
+    static_assert(!has_value_type<T>::value, "");
+    static_assert(!has_pointer<T>::value, "");
+    static_assert(!has_reference<T>::value, "");
+    static_assert(!has_iterator_category<T>::value, "");
+  }
+#if TEST_STD_VER <= 17 || !defined(__cpp_lib_concepts)
+  {
+    typedef std::iterator_traits<NotAnIteratorNoPointer> T;
+    static_assert(!has_difference_type<T>::value, "");
+    static_assert(!has_value_type<T>::value, "");
+    static_assert(!has_pointer<T>::value, "");
+    static_assert(!has_reference<T>::value, "");
+    static_assert(!has_iterator_category<T>::value, "");
+  }
+#endif // TEST_STD_VER <= 17 || !defined(__cpp_lib_concepts)
+  {
+    typedef std::iterator_traits<NotAnIteratorNoReference> T;
+    static_assert(!has_difference_type<T>::value, "");
+    static_assert(!has_value_type<T>::value, "");
+    static_assert(!has_pointer<T>::value, "");
+    static_assert(!has_reference<T>::value, "");
+    static_assert(!has_iterator_category<T>::value, "");
+  }
+
+  {
+    typedef std::iterator_traits<NotAnIteratorNoCategory> T;
+    static_assert(!has_difference_type<T>::value, "");
+    static_assert(!has_value_type<T>::value, "");
+    static_assert(!has_pointer<T>::value, "");
+    static_assert(!has_reference<T>::value, "");
+    static_assert(!has_iterator_category<T>::value, "");
+  }
+}