Do you know what cases this can return false for? If it only returns false when called with invalid arguments then we intentionally exclude it from the return type.

It looks like it happens here, but I don't know what the possibilities for INTL_DATA_ERROR_CODE((nfo)) are or why they would happen. If you can find a case on 3v4l.org that returns false for valid arguments that would be excellent.

Further investigation leads here, which possibly sets it to U_MEMORY_ALLOCATION_ERROR which I'm not sure we care about. I'm always a bit nervous when looking at current code for linked libraries though, since it's possible an older version does something different, and that's not tied directly to the PHP version.

Usually, we would remove the false return when it was related to wrong param types. I'd say the cases are the same than function that now throws a TypeError in PHP8 instead of returning false.

So if a function still return false in PHP8, I think we should keep the falsable return

You can see it's here for

'NumberFormatter::format' => ['string|false', 'num'=>'', 'type='=>'int'],

So it should be consistent and be here for formatCurrency.

It should allow to use a false check

$result = $formatter->formatCurrency(...);
if (false !== $result) {
     ...
}

which is valid php code.

Further investigation leads here, which possibly sets it to U_MEMORY_ALLOCATION_ERROR which I'm not sure we care about.

As soon as it's possible, the false check should not be reported as an error.

So if a function still return false in PHP8, I think we should keep the falsable return

100% agree.

Then if someone want to improve the return type, it should be done with a dynamic return type calculation

Thanks!