I think these are the most stripped down examples I could figure out,
and the gist of it seems to be that it has to do with splitting the
right hand side of a port map by amounts that are not known at
elaboration time...or something like that. In fails2.vhdl the
variable isn't declared anywhere, but it still crashes with the
bug notice, rather than just failing with an error message.

works.vhdl is provided for contrast, i.e. generics are used
except for the specific line that would cause it to crash.