Fix #79908: json_decode decodes negative zero as positive zero

[cmb69]

While the JSON standard knows only numbers (i.e. it doesn't distinguish
between int and float), and does not mention negative zero at all, PHP
makes that distinction, so a negative zero should be parsed as such,
even if the number is not otherwise designated as a float.

[cmb69]

PHP makes that distinction

To be clear: PHP distinguishes between int and float; it does not parse -0 as negative zero, though.

[nikic]

I'm not sure this is the right way to go about it. I think it would make more sense to special case negative zero to JSON encode as -0.0, but keep parsing of -0 as-is.

I checked the Python and Ruby JSON implementations, and they both decode -0.0 as negative zero and -0 as integer zero.

[cmb69]

Might indeed be the better solution. New fix pushed.

[bukka]

@cmb69 I'm not sure if I understand what is being fixed here. There has been explicitly RFC for this: https://wiki.php.net/rfc/json_preserve_fractional_part . Basically anyone who wants to preserve zero fraction (which logically includes -0.0 and 0.0) should use JSON_PRESERVE_ZERO_FRACTION. I think this makes it inconsistent with other numbers. Am I missing anything?

[cmb69]

@bukka, it is not about preserving the fraction, but about the sign. As it was, a negative 0 lost its sign after roundtrip: var_dump(json_decode(json_encode(-0.))); // int(0). That does not happen for other (negative) numbers.

[bukka]

And that's kind of expected because it will be int so there's no difference between -0.0 and 0.0. If you do var_dump(json_decode(json_encode(-1.)));, then it's int(-1). It just seems inconsistent that -0.0 is the only zero fraction that actually produce float. I think if someone really needs this sort of identity, they should just use JSON_PRESERVE_ZERO_FRACTION.

[cmb69]

That was my first reaction to the bug report as well; however, this case is somewhat special, since without JSON_PRESERVE_ZERO_FRACTION you never loose precision, but you loose the sign. I'm also fine with my original patch or even to close the ticket as WONTFIX. We just need consensus about what to do. :)

[bukka]

After reading the bug, I think it would make more sense to do

encode (float) (-0.0) as "0" without JSON_PRESERVE_ZERO_FRACTION
encode (float) (-0.0) as "-0.0" with JSON_PRESERVE_ZERO_FRACTION

The first part is basically what V8 (NodeJS) does:

> JSON.stringify(JSON.parse('-0.0'))
'0'

Although I don't see it as a bug so it should just target master. Unless there's something really broken (which is not in this case), changes in json encoding should not go to bug fixing releases as it might do more harm than good (e.g. might cause change of content after minor update which might not be expected).

[cmb69]

If it's not a bug, there's nothing to fix for me. I have reverted the commits, and sent a mail to the RMs to not forget to revert from PHP-7.4.22 and PHP-8.0.9 as well.