libstdc++: Make net::service_already_exists default constructible

The LWG issue I created is Tentatively Ready and proposes to declare a
public default constructor, rather than the private one I added
recently.

	* include/experimental/executor (service_already_exists): Make default
	constructor public (LWG 3414).
	* testsuite/experimental/net/execution_context/make_service.cc: Check
	the service_already_exists can be default constructed.

libstdc++-v3/ChangeLog

+2020-04-24  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/experimental/executor (service_already_exists): Make default
+	constructor public (LWG 3414).
+	* testsuite/experimental/net/execution_context/make_service.cc: Check
+	the service_already_exists can be default constructed.
+
 2020-04-24  Kamlesh Kumar  <kamleshbhalui@gmail.com>
 	    Jonathan Wakely  <jwakely@redhat.com>
 

libstdc++-v3/include/experimental/executor

@@ -129,10 +129,9 @@ inline namespace v1
 
   class service_already_exists : public logic_error
   {
-    template<typename _Service, typename... _Args>
-      friend _Service&
-      make_service(execution_context&, _Args&&...);
-
+  public:
+    // _GLIBCXX_RESOLVE_LIB_DEFECTS
+    // 3414. service_already_exists has no usable constructors
     service_already_exists() : logic_error("service already exists") { }
   };
 

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

@@ -34,3 +34,6 @@ void test01(net::execution_context& c)
 {
   net::make_service<S>(c);
 }
+
+static_assert(std::is_default_constructible<net::service_already_exists>(),
+	      "LWG 3414. service_already_exists has no usable constructors");