Bug in alias expansion

[Kynn Jones <kynnjo@xxxxxxxxx>]

The example below illustrates the problem:

#### OK VERSION 5.0.7 ####
% zsh --version
zsh 5.0.7 (x86_64-pc-linux-gnu)
% typeset -A frobozz
% alias -g foo='echo xyz'
% frobozz[$(foo)]=9
% echo ${(kv)frobozz}
xyz 9

#### FAILING VERSION 5.1 ####
% zsh --version
zsh 5.1 (i386-unknown-netbsdelf6.1)
% typeset -A frobozz
% alias -g foo='echo xyz'
% frobozz[$(foo)]=9
zsh: not an identifier: frobozz[$(fooech9


In the examples above, the code is identical in both cases, up to (and
including) the first failing command.

Failure reproduced also under zsh 5.1.1 (x86_64-debian-linux-gnu).
(Ref: http://unix.stackexchange.com/q/242730/10618)

kj