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

[Jonathan Wakely via Gcc-patches]

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.

Tested powerpc64le-linux, committed to master.

I'll backport it to gcc-9 too.


commit c62f5e6e1f457462b1cea74792833821bbea64bb
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Mon Mar 16 22:53:42 2020 +0000

    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.

diff --git a/libstdc++-v3/include/experimental/executor 
b/libstdc++-v3/include/experimental/executor
index 5577623bb47..6955ead6bf5 100644
--- a/libstdc++-v3/include/experimental/executor
+++ b/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;
 
diff --git 
a/libstdc++-v3/testsuite/experimental/net/execution_context/make_service.cc 
b/libstdc++-v3/testsuite/experimental/net/execution_context/make_service.cc
new file mode 100644
index 00000000000..f134add48b5
--- /dev/null
+++ b/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);
+}