libstdc++: Add C++11 member functions for ios::failure in old ABI

The new constructors that C++11 added to std::ios_base::failure were
missing for the old ABI. This adds them, but just ignores the
std::error_code argument (because there's nowhere to store it).

This also adds a code() member, which should be provided by the
std::system_error base class, but that base class isn't present in the
old ABI.

This allows the old ios::failure to be used in code that expects the new
API, although with reduced functionality.

libstdc++-v3/ChangeLog:

	* include/bits/ios_base.h (ios_base::failure): Add constructors
	takeing error_code argument. Add code() member function.
	* testsuite/27_io/ios_base/failure/cxx11.cc: Allow test to
	run for the old ABI but do not check for derivation from
	std::system_error.
	* testsuite/27_io/ios_base/failure/error_code.cc: New test.

libstdc++-v3/include/bits/ios_base.h

@@ -289,6 +289,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       virtual const char*
       what() const throw();
 
+#if __cplusplus >= 201103L
+      // Define the new members required by C++11,
+      // even though the error_code cannot be stored.
+
+      explicit
+      failure(const string& __s, const error_code&) noexcept
+      : failure(__s)
+      { }
+
+      explicit
+      failure(const char* __s, const error_code& = error_code{})
+      : failure(string(__s))
+      { }
+
+      // Stand-in for system_error::code() but returning by value.
+      error_code code() const noexcept { return error_code{}; }
+#endif
+
     private:
       string _M_msg;
     };

libstdc++-v3/testsuite/27_io/ios_base/failure/cxx11.cc

@@ -16,14 +16,15 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-do run { target c++11 } }
-// { dg-require-effective-target cxx11-abi }
 
 #include <ios>
 #include <testsuite_hooks.h>
 
 using test_type = std::ios_base::failure;
 
+#if _GLIBCXX_USE_CXX11_ABI
 static_assert( std::is_base_of<std::system_error, test_type>::value, "base" );
+#endif
 
 void
 test01()

libstdc++-v3/testsuite/27_io/ios_base/failure/error_code.cc

@@ -0,0 +1,48 @@
+// 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-do run { target c++11 } }
+
+#include <ios>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::error_code ec, def_ec;
+#if _GLIBCXX_USE_CXX11_ABI
+  // For the new ABI code() should return the constructor argument.
+  ec = std::make_error_code(std::errc::executable_format_error);
+  def_ec = std::io_errc::stream;
+#else
+  // For the old ABI code() always returns a default-constructed error_code.
+#endif
+  std::ios_base::failure e1("string literal");
+  VERIFY( e1.code() == def_ec );
+  std::ios_base::failure e2(std::string("std::string"));
+  VERIFY( e2.code() == def_ec );
+  std::ios_base::failure e3("string literal", ec);
+  VERIFY( e3.code() == ec );
+  std::ios_base::failure e4(std::string("std::string"), ec);
+  VERIFY( e4.code() == ec );
+}
+
+int
+main()
+{
+  test01();
+}