sdbusplus::asio: fix callback with void args specialization

Callbacks with no arguments failed to build due to an error in the
utility::get_first_arg template. It failed to provide a type member for
the void specialization. With this fix, it is possible to have a handler
that has no arguments.

Tested by: Added a new callback to the asio-example.cpp. Without the
           template change, this caused the build to fail as expected.
           After adding the template change, the build succeeds and the
           example runs as expected.

Change-Id: Ib09b58c76df8cba57dc093ab37a4c1b68ea11ad3
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/example/asio-example.cpp b/example/asio-example.cpp
index 31b55f2..354c7b0 100644
--- a/example/asio-example.cpp
+++ b/example/asio-example.cpp
@@ -17,6 +17,11 @@ int methodWithMessage(sdbusplus::message::message& m, int test)
     return ++test;
 }
 
+int voidBar(void)
+{
+    return 42;
+}
+
 int main()
 {
     using GetSubTreeType = std::vector<std::pair<
@@ -121,6 +126,8 @@ int main()
 
     iface->register_method("TestMethodWithMessage", methodWithMessage);
 
+    iface->register_method("VoidFunctionReturnsInt", voidBar);
+
     iface->initialize();
     iface->set_property("int", 45);
     io.run();