return type constraints match undefined values

Author: gfldex

doc/Type/Signature.pod

@@ -195,10 +195,13 @@ the signature after the argument name.
 The token C<-->> followed by a type will force a type check on successful
 execution of a routine. The return type arrow has to be placed at the end of
 the parameter list. The keyword C<returns> following a signature declaration
-has the same function. C<Nil> is considered as being defined for the type check.
+has the same function. C<Nil> is considered as being defined for the type
+check. Type objects and thus undefined values will pass the test unless the
+type smiley C<:D> is used.
 
 
-	sub foo(--> Int) { 1 };
+	sub foo(--> Int) { my Int $i; $i };
+    sub foo(--> Int:D) { 1 }
 	sub foo() returns Int { 1 };
 	sub does-not-work(--> Int) { "" }; # throws X::TypeCheck::Return