functions being defined (they can still be included if quoted).
If we parsed the way POSIX specifies (leaving the exact input text of
$ and ` expansions unaltered, until required to be expanded) this would
not be needed, as the name of a function being defined does not underbo
parameter, command, or arith expansions, so xxx$3() { : ; } would just
work. But for many reasons we don't do that (and are unlikely to ever,
though maintaing both forms might be an option someday) - which led to
very obscure behaviour (if sh were compiled in DEBUG mode, even an abort())
and certainly nothing useful. So just prohibit these uses for now.
(A portable function name must be a "name" so this makes no difference
at all to posix compat applications/scripts).
A doc update is pending (the updated sh.1 also contains updates in other
areas not yet appropriate to commit).
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