Expression binding <:: and output! can be misleading

[sylefeb]

Expression tracking may use the 'value at previous clock' assignment (::=). However, in some cases (tracking another tracker, or value bound to instance output) this does nothing as there is no flip-flop associated. In such a case ::= is the same as :=.

This should issue an error. The docummentation should explain and provide a solution on how to achieve the desired effect (i.e. adding an intermediate flip-flop).

[sylefeb]

A similar situation exists with computational outputs (output!) where this does no make any difference when the output is replaced by a wire (bound).

[sylefeb]

Expression tracking is solved for the case of tracking another tracker, disallowing mixing ::= and :=
The case with outputs remains open (tracking an output; chaining outputs).

[sylefeb]

Note that expression tracking changed terminology for 'expression binding' and now uses the operators <: and <::. The issue remains.

[sylefeb]

Closing as this is now clarified and warnings are issued on such cases.