[lazerg]

The DFA pass turns $x = $x - 1 into PRE_DEC when the operand can only be null, undef or a number. That is fine for + 1, but not for - 1: null - 1 is -1, while --$x on null leaves it null. The ASSIGN_OP SUB rule right below already excludes MAY_BE_UNDEF|MAY_BE_NULL, so the two SUB contraction rules now do the same.

Fixes GH-25061

[arshidkv12]

Thank you for your PR. Could you also check the following case?

function d5(?int $x) {
    $x--;
    return $x;
}

var_dump(d5(null));

Currently, this produces the following warning:

Warning: Decrement on type null has no effect, this will change in the next major version of PHP

Should this case also be handled?

[lazerg]

I left that case alone because it is the intended behavior. $x-- on null gives the same warning and NULL without opcache too, since decrementing null has been a no-op with that warning since 8.3. This PR only fixes the optimizer turning $x = $x - 1 into a decrement, which changed the result.

[arshidkv12]

Thanks for the clarification. This PR only addresses the incorrect optimization that changes the result.

I think a separate PR would be needed to address the behavior of $x = null; $x--; in line with the Saner Increment/Decrement Operators RFC.

[ndossche]

This is fine, but add a comment to the edited lines that the MAY_BE_UNDEF|MAY_BE_NULL exists because of the currently inconsistent behaviour and that in PHP9 we should revisit this once the behaviour is fixed

[lazerg]

Added the comment on both edited lines.

[ndossche]

Thanks