libstdc++: Define std::bad_optional_access constructor as defaulted

The standard requires that std::bad_optional_access' default
constructor has a non-throwing exception specification.

	* include/std/optional (bad_optional_access): Define default
	constructor and destructor as defaulted.
	* testsuite/20_util/optional/bad_access.cc: New test.

libstdc++-v3/include/std/optional

@@ -84,12 +84,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   class bad_optional_access : public exception
   {
   public:
-    bad_optional_access() { }
+    bad_optional_access() = default;
+    virtual ~bad_optional_access() = default;
 
-    virtual const char* what() const noexcept override
+    const char* what() const noexcept override
     { return "bad optional access"; }
-
-    virtual ~bad_optional_access() noexcept = default;
   };
 
   void

libstdc++-v3/testsuite/20_util/optional/bad_access.cc

@@ -0,0 +1,27 @@
+// 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++17" }
+// { dg-do compile { target c++17 } }
+
+#include <optional>
+
+using std::bad_optional_access;
+
+static_assert( std::is_nothrow_default_constructible_v<bad_optional_access> );
+static_assert( std::is_nothrow_copy_constructible_v<bad_optional_access> );
+static_assert( std::is_nothrow_copy_assignable_v<bad_optional_access> );