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What guarantees does Bash make about order of :- Parameter Expansion when it is not in POSIX mode?

What guarantees does Bash make about order of :- Parameter Expansion when it is not in POSIX mode?

I tried to check `info bash` but only got a copy of the `man` page. Which is when I learned that I could `apt install bash-doc` to get the "full" [Bash manual.](https://www.gnu.org/software/bash/manual/bash.html)

After checking both the info pages and the man page, I found this is pretty much the only description of **Parameter Expansion:**

>    In each of the cases below, WORD is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. 
>    
>    When not performing *substring expansion,* using the form described below (e.g., `':-'`), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both PARAMETER's existence and that its value is not null; if the colon is omitted, the operator tests only for existence.  
>   
> `${PARAMETER:-WORD}`  
>      If PARAMETER is unset or null, the expansion of WORD is substituted.  Otherwise, the value of PARAMETER is substituted.  
> 
>         $ v=123  
>         $ echo ${v-unset}  
>         123

If I write `${v:-$(some-command)}` the documentation does not specify whether `some-command` is run or not. In fact, the documentation seems to me to hint that **WORD** will be expanded whether I want it to be, or not. 

I tried some fairly obvious experiments:

    $ echo $BASH_VERSION
    5.2.15(1)-release
    $ something() { echo "something" > /dev/tty; echo "anything" ; }
    $ echo ${no_such_var:-$(something)}
    something
    anything
    $ x=1
    $ echo ${x:-$(something)}
    1
    $ echo ${no_such_var:-$(something)}
    something
    anything

This seems to indicate that this particular version of Bash do not expand the **WORD** part of the Parameter Expansion unless it is needed. 

What I am looking for is a guarantee. [POSIX says](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02):

> In each case that a value of *word* is needed (based on the state of *parameter,* as described below), *word* shall be subjected to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If *word* is not needed, it shall not be expanded. 

And Bash promises that it will behave in a compliant fashion when instructed to do so. (Running as `sh`, passed `--posix`, configured with --enable-strict-posix-dammit-i-really-mean-it-this-time, etc.) But when Bash is running with all the "extra goodness" in not-posix mode, the documentation appears to be silent. This seems like an important point to omit from the documentation, and one where copying the text from the POSIX spec would have been fairly simple. 

My question, then, is this: what guarantees does Bash make about order of :- Parameter Expansion when it is not in POSIX mode?