David Blaikie suggested this test for detecting constexpr on the error_category default constructor. Additionally I decided to make this constructor trivial as well, as I could not come up with a rationale for not doing so. Fully tested with both current Apple clang and tip-of-trunk clang.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189034 91177308-0d34-0410-b5e6-96231b3b80d8

test/diagnostics/syserr/syserr.errcat/syserr.errcat.nonvirtuals/default_ctor.pass.cpp

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <system_error>
+
+// class error_category
+
+// constexpr error_category() noexcept;
+
+#include <system_error>
+#include <type_traits>
+#include <string>
+#include <cassert>
+
+#if _LIBCPP_STD_VER > 11
+
+class test1
+    : public std::error_category
+{
+public:
+    constexpr test1() = default;  // won't compile if error_category() is not constexpr
+    virtual const char* name() const noexcept {return nullptr;}
+    virtual std::string message(int ev) const {return std::string();}
+};
+
+#endif  // _LIBCPP_STD_VER > 11
+
+int main()
+{
+#if _LIBCPP_STD_VER > 11
+    static_assert(std::is_nothrow_default_constructible<test1>::value,
+                                 "error_category() must exist and be noexcept");
+#endif  // _LIBCPP_STD_VER > 11
+}