${a:=b} expands to `b', not `a''s value

[Oğuz]

    $ declare -l a
    $ echo "${a:=X} $a"
    X x

This doesn't jive with what the manual says.

`-l`:
> When the variable is assigned a value, all upper-case characters are
converted to lower-case.

`:=`:
> If parameter is unset or null, the expansion of word is assigned to
parameter. The value of parameter is then substituted.

Is this a bug or am I missing something here?


-- 
Oğuz