[ondrejmirtes]

Closes phpstan/phpstan#15727

Since bcca2b0 ("Narrow remembered count()/sizeof() of an array when the array expression is narrowed", #6953), mergeWith() drops a remembered call whose merged type equals what the call resolves to on its own. The goal is to let later narrowing of an argument reach the call: count($arr) after $arr !== []. But dropping the entry is not neutral for later merges:

• "Might not be defined" (WordPress): after if (isMulti()) {}, the scope no longer remembers isMulti(): bool. In if (!isMulti() || can('update')), the left operand's scope then has no entry, so the || merge leaves a Maybe holder for isMulti() in the truthy branch. A Maybe holder derives no conditional guard, so the merge after the if loses "if isMulti() is false, $updateData is defined".
• Call narrowed after an if without else (Dolibarr): the same Maybe holder (non-falsy-string from the right operand of getS() && …) is intersected as if it held the call's type when the conditional expression "if $client is null, getS() is ''|'0'" fires. That makes the else branch never, and getS() ends up non-falsy-string after the if.

Only an argument that narrowing can change can make a remembered call stale. A call without one now keeps its entry, as it did in 2.3.5. This covers calls with no arguments and calls whose arguments are all literals: scalars other than interpolated strings, constants, and class constants of a named class. count($arr), strlen($s) and the rest of bug-13750.php are still dropped as before.

The turbo-ext mirror in MutatingScope.cpp gets the same check, and generated/MutatingScope.h is regenerated with PHP 8.5.

Not covered: the same pattern on a call with a narrowable argument, such as if (f($i)) {} followed by if (!f($i) || …), still reports "might not be defined". It behaves the same way as on 2.3.x-dev today, and keeping that entry would undo #6953 for that call.

Verification:

• tests/PHPStan/Analyser/nsrt/bug-15727.php and DefinedVariableRuleTest::testBug15727() fail without the fix (4 failing assertions plus the rule error on line 30) and pass with it. They also pass with the extension loaded and only the C++ change in place.
• The full test suite passes with and without the extension (23702 tests). make phpstan reports no errors with and without the extension. The strict extension build, smoke.php, signature-parity.php and side-by-side.php pass.
• Not run: make lint-turbo, because clang-tidy 21 is not available here.