Bug in parameter expansion with :- ?

[Ronald Fischer <ynnor@xxxxx>]

This expands correctly to a '+':

      ${unset_variable:-+}

This however doesn't expand to a '-', but to the empty string:

    ${unset_variable:--}

I didn't find anything in the man page which would explain this
behaviour.

A note aside: Bash expands in the latter case to a dash (-). 

My zsh version:

zsh 5.1.1 (x86_64-unknown-cygwin)


Ronald