Fix #6777: Bogus write-only error when appending to an array-like object in private class property

[phpstan-bot]

Summary

When appending to an ArrayAccess object stored in a private class property (e.g. $this->array[] = $s), PHPStan incorrectly reported "Property is never read, only written." This is a false positive because appending to an ArrayAccess object is semantically $this->array->offsetSet(null, $s) — a method call that reads the property.

Changes

• Modified src/Node/ClassStatementsGatherer.php: When handling a PropertyAssignNode, check if the assigned expression is a SetOffsetValueTypeExpr or SetExistingOffsetValueTypeExpr (indicating array dim fetch assignment) and the property type is an ArrayAccess object. If so, also record a PropertyRead.
• Updated existing WriteToCollection test expectations in tests/PHPStan/Rules/DeadCode/UnusedPrivatePropertyRuleTest.php to reflect the fix.
• Added regression test in tests/PHPStan/Rules/DeadCode/data/bug-6777.php.

Root cause

In ClassStatementsGatherer, when NodeScopeResolver processes $this->array[] = $s on an ArrayAccess property, it emits a PropertyAssignNode which was unconditionally recorded as a PropertyWrite. Meanwhile, the PropertyFetch for $this->array was processed in expression-assign mode, causing ClassStatementsGatherer to skip recording a PropertyRead. This resulted in the property being tracked as "only written" even though it was being read to call offsetSet() on the object.

The fix detects when a property assignment comes from an array dim fetch operation on an ArrayAccess object and also records a PropertyRead, since the object must be read from the property to invoke the method.

Test

Added tests/PHPStan/Rules/DeadCode/data/bug-6777.php which reproduces the original issue: a class with a private ArrayObject property that is only appended to via $this->array[] = $s. The test expects no errors.

Fixes phpstan/phpstan#6777