Infinite recursions in math evaluation

[Mikael Magnusson <mikachu@gmail.com>]

I was trying to do something completely different and noticed that
this causes an infinite recursion:
a=a; ${(#)a}
and this doesn't
${(#):-a}
(and surprised me by cding into my first $cdpath entry, turns out it
returned a null character, and that does that with autocd on.)

The thing seems to be the variable name having the same name as the
value of it. Aha, actually a=a; $(( a )) crashes in the same way. (On
a machine with a presumably less optimized/buggy compile, it does
print "zsh: math recursion limit exceeded" instead of crashing). Is it
simply a user error? I wouldn't expect either of these expressions to
recursively look up values of the parameter until it encountered a
number, but this is indeed what happens:
% a=b; b=c; c=d; d=e; e=f; f=5; echo $(( a ))
5
bash does the same...

It seems to be of somewhat random utility though:
a='b c'
b='5 +'
c='5*3'
echo $(( a ))
zsh: bad math expression: operator expected at `c'
a='b+c'
echo $(( a ))
zsh: bad math expression: operand expected at `'
b='55+32'
echo $(( a ))
102

I scanned quickly through the "Arithmetic Evaluation" section and
found nothing suggesting that this should happen. It is also perhaps
somewhat unclear from the description of the (#) flag in the manpage
that it causes the same eval as in $(( )), it simply says the
resulting words are parsed as numeric expressions. Executing echo
${(#):-'foo=bar, bar=65, foo'} twice outputs A the second time btw.

I would not be opposed to just describing this behaviour in the
manpage (if I didn't just miss a huge section that describes it
again.. :)

-- 
Mikael Magnusson

PS What I originally actually wanted to run was $(( #a )), but I
remembered the wrong syntax.