JIT unsoundness with recursive function [in 8.6beta3 and 8.5.x] #23693

<?php
declare(strict_types=1);

function checkWord(string $board, string $word, int $pos, int $idxChar = 0, int $usedBitboard = 0): bool
{
    error_log("checkWord($board, $word, $pos, $idxChar, $usedBitboard)");
    if ($pos < 0) {
        error_log("--- pos = $pos");
        throw new \Exception('impossible, $pos cannot be negative, yet it is.');
    }
    if ($board[$pos] != $word[$idxChar]) {
        return false;
    }
    if ($idxChar == strlen($word) - 1) {
        return true;
    }
    $newUsedBitboard = $usedBitboard | (1 << $pos);
    $x = $pos & 0b11;
    $y = $pos >> 2;
    for ($dy = -1; $dy <= 1; ++$dy) {
        for ($dx = -1; $dx <= 1; ++$dx) {
            if (($dx == 0) and ($dy == 0)) {
                continue;
            }
            $newX = $x + $dx;
            $newY = $y + $dy;
            if (($newX < 0) or ($newX >= 4) or ($newY < 0) or ($newY >= 4)) {
                continue;
            }
            $newPos = ($newY << 2) | $newX;
            if (($newUsedBitboard & (1 << $newPos)) != 0) {
                continue;
            }
            if (checkWord($board, $word, $newPos, $idxChar + 1, $newUsedBitboard)) {
                return true;
            }
        }
    }
    return false;
}

// ---------- main program
$board = 'RANRTHCTBTEAEOTN';
$notepad_input = [
    'BETTER HEN EACH TORT TOE CANT TEA HEAT HAT TRACE TRACT TOT TAR ETA TAR TAN TOTE CHART TENT CAT CHART TOTE TEA ACE ART TRANCE TOT CANE CHART CAN CHAT CANT THEN NET TRACE BOTH HERE',
    'BOTH CHAT EACH CAT CENT CANE TEA CART ANTE TORCH HET RANT ACT TOE ACT HAT TOE CHAT BOTH CANE CART BOTH TEACH EACH NET ACE TEA BET THAT BETH ACT EACH RAT EAR CHEAT ETA TAN ACT CANE ANT TECH TENT EAT TEA ATE CAN THAT THETA',
    'HEAT HAT TOTE NEAT RANCH CAN THEN THEN THE BET ATE RAN THAT TOTE TENT EACH CENT TERRACE HAT CAR TEA BOTH CAT CENT CAR BETH ANT HEN HEAT CENT TECH ANT BOTH',
    'CANT ANT TRACT THAT THEN CHAR TRACE THAT HEAT RAT RAN HAT CAR THE THETA CHEAT CHART ETA ANCHOR',
];
$notepads = [];
$wordToWriter = [];
for ($i = 0; $i < 4; ++$i) {
    $notepads[$i] = explode(' ', $notepad_input[$i]);
    foreach ($notepads[$i] as $j => $word) {
        if (!isset($wordToWriter[$word])) {
            $wordToWriter[$word] = [$i => [$j]];
        } elseif (!isset($wordToWriter[$word][$i])) {
            $wordToWriter[$word][$i] = [$j];
        } else {
            $wordToWriter[$word][$i][] = $j;
        }
    }
}
// in the loop below sometimes Exception thrown when invoking: checkWord($board, 'TAR', 4);
for ($i = 0; $i < 4; ++$i) {
    foreach ($notepads[$i] as $j => $word) {
        if ((count($wordToWriter[$word] ?? []) <= 1) && ($wordToWriter[$word][$i][0] == $j)) {
            for ($pos = 0; $pos < 16; ++$pos) {
                if (checkWord($board, $word, $pos)) {
                    break;
                }
            }
        }
    }
}
echo '-- it works, no Exception thrown' . PHP_EOL;