Suppose you define OhHai as a synonym for a{us} as follows:

  <tp:mapping name="Oh_Hai">
    <tp:member name="u" name="Contact"/>
    <tp:member name="s" name="How_Exciting"/>
  </tp:mapping>

And then you annotate a method argument as having that type, but get
the D-Bus type wrong:

  <method name="GetParty" tp:name-for-bindings="Get_Party">
    <arg direction="in" name="Wear_Party" type="au"/>
    <arg direction="out" name="Fly"
    	 type="a{ss}" tp:type="Oh_Hai"/>
    <!-- whoops  ^ -->
  </method>

I'd expect the doc generator to complain, but it did not.