Bug Report

The rule ships in PHPUnitSetList::PHPUNIT_NARROW_ASSERTS and PHPUnitSetList::PHPUNIT_CODE_QUALITY, which is applied set-wide, and the failure mode is a green suite turning red with an assertion that looks deliberate (Failed asserting that 100 is identical to '100'). It silently broke 4 tests across 4 files in our codebase.

Minimal PHP Code Causing Issue

Reproduced the issue here.

PHPStan memoizes the narrowing against the call expression, and the next, possibly-impure setTypeId() does not invalidate it (rememberPossiblyImpureFunctionValues, on by default). So getNativeType() answers string, isString()->yes() passes, and the literal is rewritten. The declared type int|string|null would have answered maybe and correctly bailed.

The single preceding assertSame() is the entire trigger, so a byte-identical assertion in a method without it is left alone.

Expected Behaviour

I have created a draft PR to demonstrate a possible fix rectorphp/rector-phpunit#771