What if you disable opcache?

Indeed, disabling opcache fixed the problem. So is this a bug specific to 7.3 or the windows version? Because I never had a problem with 7.2 and opcache enabled.

I doubt it's specific to Windows. Likely just a regular opcache bug.

Can you please provide the (non-reduced) source code of the function that is failing? If you can't publish the code, please feel free to mail to me directly at nikic@php.net.

I'm really sorry, but I can't. I'm working on a backend for a music publisher and we're talking about a huge project. When I find the time I'll try again to reproduce the problem outside this project. It doesn't happen if I isolate the method so it could be hard to pinpoint... :(

For this purpose it's usually enough to only have the function that contains the switch, even if it cannot be run in isolation. The surrounding code (likely) does not matter. The code is only needed to get an opcode dump and see the faulty transformation, a smaller test case can then be reconstructed from there.

Reduced test case based on code provided via mail:

function test($x) {
    switch ($x["y"]) {
        case "a":
            break;
        case "b":
            break;
    }
}

This generates "Block 4 predecessors missing 1" SSA verification errors, first appearing after jump optimization.

Okay, that was maybe a bit too reduced. A better variant is:

<?php
function test($x) {
    $a = false;
    switch($x["y"]) {
        case "a":
            $a = true;
            break;
        case "b":
            break;
        case "c":
            break;
    }
    return $a;
}
var_dump(test(["y" => "a"]));

The issue is basically that we currently require that block predecessors are unique. The code for unlinking a block has a broken implementation of the deduplication. However, the more important issue is that if we remove predecessors due to deduplication, we must also adjust phi nodes accordingly.

Those issues can be fixed, but I think that the cleaner fix would be to relax the restriction on duplicate predecessors. Not sure what other fallout that would have though.