UML22 — UML 2 Issue: Message notation

PROBLEM STATEMENT
According to Superstructure, p. 430, the notation for messages in
interaction diagrams is as follows (we put assumptions between parenthesis):

asynchronous message - (solid line) open arrowhead
synchronous message - (solid line) filled arrowhead
reply message - dashed line (filled or open arrowhead?)
object creation - dashed line open arrowhead

However, the example given in Figure 333, p. 414, shows a different
notation:

asynchronous message - solid line open arrowhead (not shown in this diagram,
but in others)
synchronous message - solid line filled arrowhead
reply message - dashed line OPEN arrowhead
object creation - SOLID line open arrowhead

Another confusing aspect of the notation is that in a reply message, which
is not a true message, the message name is the name of the operation invoked
on the callee (same as in the corresponding synchronous call), but it
suggests instead that there is an operation with this name in the caller. In
Figure 333, the reply labeled as "foo(_)" visually suggests that there is an
operation named foo in class C1, which is wrong: foo is defined in C2, not
in C1. It would make more sense to label a reply message with the name of
the value returned.

PROPOSED SOLUTION
The simplest solution would be to fix Figure 333 using a dashed arrow to
represent object creation, although this would yield the same notation for
object creation and reply message. Therefore, beyond this simplest solution,
we propose something more advanced: First, state explicitly the notation for
all kinds of messages, leaving no place for assumptions. Second, use a
filled arrowhead for reply messages, since this emphasizes the conceptual
proximity to the synchronous message it is a reply from. Third, use the
notation for object creation also for object deletion, which currently is
not mentioned. That is:

asynchronous message - SOLID LINE open arrowhead
synchronous message - SOLID LINE filled arrowhead
reply message - dashed line FILLED ARROWHEAD
object creation OR DELETION - dashed line open arrowhead

Or better, simply drop object creation as an special kind of message. Object
creation (and deletion) was not considered a special kind of message in UML
1, and it is not at all clear why it should be in UML 2. Probably, an object
creation is either synchronous or asynchronous, but not "something else" in
the same meta-specialization row. In fact, the constraints and semantics of
Message (Superstructure, p. 429) do not consider object creation as
messages: "The signature must either refer an Operation (...) or a Signal",
"A Message reflects either an Operation call (...) or a sending and
reception of a Signal". Neither does the meta-attribute Message.messageSort,
which has the following permitted values: synchCall, synchSignal,
asynchCall, asynchSignal. By the way, what do synchSignal and asynchCall
mean? The "sorts" of message are not defined in the Spec. Although calls are
considered in other places to be either synchronous or asynchronous, signals
are explicitly defined to be asynchronous (Superstructure, pp. 15, 371, 394
and 395), therefore at least synchSignal is banned.

Finally, we also propose to label reply messages with the name of the value
returned by the operation call, not the operation name itself. In Figure
333, this would leave the replies foo() and doit() without label, and the
last reply would be labeled simply as "x".

see above