libstdc++: Mark experimental::net::system_context ctor deleted

           * include/experimental/net/executor (system_context): Mark
           system_context::system_context() = delete.
           * testsuite/experimental/net/executor/1.cc: Add new
           test to check system_context is not default constructible.

libstdc++-v3/ChangeLog

@@ -14,6 +14,13 @@
 	* include/std/version (__cpp_lib_three_way_comparison): Define for
 	freestanding builds.
 
+2020-04-23  Thomas Rodgers  <rodgert@appliantology.com>
+
+	* include/experimental/net/executor (system_context): Mark
+	system_context::system_context() = delete.
+	* testsuite/experimental/net/executor/1.cc: Add new
+	test to check system_context is not default constructible.
+
 2020-04-23  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* include/std/coroutine: Update the inline namespace to __n4861.

libstdc++-v3/include/experimental/executor

@@ -850,7 +850,7 @@ inline namespace v1
 
     // construct / copy / destroy:
 
-    system_context() = default;
+    system_context() = delete;
     system_context(const system_context&) = delete;
     system_context& operator=(const system_context&) = delete;
 

libstdc++-v3/testsuite/experimental/net/executor/1.cc

@@ -85,9 +85,16 @@ test02()
   VERIFY( e == g );
 }
 
+void
+test03()
+{
+  static_assert( ! std::is_default_constructible<net::system_context>::value, "" );
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }