BUG? - 4.0.2 - parameter substitution won't double backslashes in values

[Derek Peschel <dpeschel@xxxxxxxxxx>]

I want to write the elements of $dirstack out to a file, separated by
newlines.  If an element in $dirstack contains a newline, I want to write
a backslash before the newline in the file.  Parameter substitution
managed that:

print ${dirstack[0]/
/\\\\
}

But I also want to double any backslashes in $dirstack, and I haven't
managed to do that yet with parameter substitution.  The backslash
sequences in the parameter seem to be interpreted before substitution
happens.

Suppose I have a subdirectory "a\bc" under my home directory.

print $dirstack[0]
/usr/home/dpeschel/ac
["\b" is a backspace]

print ${dirstack[0]/\\/\\\\}
/usr/home/dpeschel/ac
[no change]

print ${dirstack[0]/\\\\/\\\\\\\\}
/usr/home/dpeschel/ac
[no change]

print ${dirstack[0]/b/t}
/usr/home/dpeschel/a	c
[the "\b" gets changed to "\t" which is a tab]

Then I thought of using a single backslash -- given that escape sequences
happen "at a lower level" than parameter substitution, and parameter-
substitution backslashes must be quoted, it makes a kind of sense that
an unquoted backslash would affect the version of the value with un-
processed escape sequences.

The result was really weird:
print ${dirstack[0]/\/t}
t/usr/home/dpeschel/ac

If there's another way to do this, aside from parameter substitution,
that would be OK.

I haven't yet tackled reading the items back from the file, but obviously
writing has to work first.

-- Derek