Filename expansion is performed on the right hand side of a parameter
assignment, including those appearing after commands of the
typeset family. In this case, the right hand side will be treated
as a colon-separated list in the manner of the PATH parameter,
so that a ‘~’ or an ‘=’ following a ‘:’ is eligible for expansion.
All such behaviour can be
disabled by quoting the ‘~’, the ‘=’, or the whole expression (but not
simply the colon); the EQUALS option is also respected.
If the option MAGIC_EQUAL_SUBST is set, any unquoted shell
argument in the form ‘identifier=expression’ becomes eligible
for file expansion as described in the previous paragraph. Quoting the
first ‘=’ also inhibits this.