libstdc++: Add new testcase for comparison category types

Comparing a comparison category type to anything except a literal 0 is
undefined. This verifies that at least some misuses are diagnosed at
compile time.

	* testsuite/18_support/comparisons/categories/zero_neg.cc: New test.

1 files changed, 46 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/18_support/comparisons/categories/zero_neg.cc b/libstdc++-v3/testsuite/18_support/comparisons/categories/zero_neg.cc
new file mode 100644
index 0000000..0a8af41
--- /dev/null
+++ b/libstdc++-v3/testsuite/18_support/comparisons/categories/zero_neg.cc
@@ -0,0 +1,46 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <compare>
+
+// C++20 [cmp.categories.pre]
+// "an argument other than a literal 0 is undefined"
+
+void
+test01()
+{
+  std::partial_ordering::equivalent == 0; // OK
+  std::weak_ordering::equivalent == 0;    // OK
+  std::strong_ordering::equivalent == 0;  // OK
+
+  std::partial_ordering::equivalent == 1; // { dg-error "invalid conversion" }
+  std::weak_ordering::equivalent == 1;    // { dg-error "invalid conversion" }
+  std::strong_ordering::equivalent == 1;  // { dg-error "invalid conversion" }
+
+  constexpr void* p = nullptr;
+  std::partial_ordering::equivalent == p; // { dg-error "invalid conversion" }
+  std::weak_ordering::equivalent == p;    // { dg-error "invalid conversion" }
+  std::strong_ordering::equivalent == p;  // { dg-error "invalid conversion" }
+
+  // Ideally these would be ill-formed, but the current code accepts it.
+  std::partial_ordering::equivalent == nullptr;
+  std::weak_ordering::equivalent == nullptr;
+  std::strong_ordering::equivalent == nullptr;
+}