libstdc++: Add default constructor to net::service_already_exists (PR 94199)

The service_already_exists exception type specified in the TS doesn't
have any constructors defined. Since its base class isn't default
constructible, that means has no usable constructors. This may be a
defect in the TS.

This patch fixes it by adding a default constructor, but making it
private. The make_service function is declared as a friend to be able to
call that private constructor.

	PR libstdc++/94199
	* include/experimental/executor (service_already_exists): Add default
	constructor. Declare make_service to be a friend.
	* testsuite/experimental/net/execution_context/make_service.cc: New
	test.

libstdc++-v3/ChangeLog

@@ -1,3 +1,11 @@
+2020-03-16  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/94199
+	* include/experimental/executor (service_already_exists): Add default
+	constructor. Declare make_service to be a friend.
+	* testsuite/experimental/net/execution_context/make_service.cc: New
+	test.
+
 2020-03-12  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/tr1/8_c_compatibility/cstdlib/functions.cc: Do not pass

libstdc++-v3/include/experimental/executor

@@ -127,7 +127,14 @@ inline namespace v1
   /// An extensible, type-safe, polymorphic set of services.
   class execution_context;
 
-  class service_already_exists : public logic_error { };
+  class service_already_exists : public logic_error
+  {
+    template<typename _Service, typename... _Args>
+      friend _Service&
+      make_service(execution_context&, _Args&&...);
+
+    service_already_exists() : logic_error("service already exists") { }
+  };
 
   template<typename _Tp> struct is_executor;
 

libstdc++-v3/testsuite/experimental/net/execution_context/make_service.cc

@@ -0,0 +1,36 @@
+// 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 compile { target c++14 } }
+
+#include <experimental/executor>
+
+namespace net = std::experimental::net;
+
+struct S : net::execution_context::service
+{
+  using key_type = S;
+
+  S(net::execution_context&);
+
+  void shutdown() noexcept override { }
+};
+
+void test01(net::execution_context& c)
+{
+  net::make_service<S>(c);
+}