%collapse crashes without give any useful diagnostic on designs containing DFFs

[EPTansuo]

Hello,

When I run the following commands:

%yosys test.v  #test.v contains DFFs
%collapse -T <top_module>

ABC hits an assertion failure in Rtl_NtkBlastOperator():

assert (Vec_IntSize(vRes) > 0)

This happens because %collapse does not support sequential designs. However, the current behavior is a hard crash and doesn't give any useful diagnostic. Could we add a clearer error message instead? E.g.:

“Sequential designs are not supported by %collapse.”

This would make the failure mode much more user-friendly for those who are not familar with this command.