Summary

When iterating a list<mixed> with foreach ($list as &$value) (without a key variable) and modifying $value, PHPStan incorrectly degraded the type to array<int<0, max>, mixed>, losing the list property. The same loop with foreach ($list as $key => &$value) correctly preserved the list type.

Changes

• Changed SetOffsetValueTypeExpr to SetExistingOffsetValueTypeExpr in MutatingScope::enterForeach() at src/Analyser/MutatingScope.php:3026 for the no-key by-reference case
• Added regression test in tests/PHPStan/Analyser/nsrt/bug-13809.php
• Updated CLAUDE.md with documentation about the SetExistingOffsetValueTypeExpr vs SetOffsetValueTypeExpr distinction in foreach context

Root cause

In MutatingScope::enterForeach(), when creating the IntertwinedVariableByReferenceWithExpr for foreach ($list as &$value) (no key variable), the code used SetOffsetValueTypeExpr to represent the array modification. This expression calls setOffsetValueType() on the array type, which in AccessoryArrayListType returns ErrorType for non-null/non-zero offset types (like int<0, max>). The ErrorType result destroys the list accessory type during intersection resolution.

The fix uses SetExistingOffsetValueTypeExpr instead, which calls setExistingOffsetValueType(). This is semantically correct because a foreach loop only modifies values at existing offsets, never adding new ones. AccessoryArrayListType::setExistingOffsetValueType() returns $this, correctly preserving the list type.

Test

The regression test covers:

• foreach ($list as &$value) without key — verifies list type is preserved (was the reported bug)
• foreach ($list as $key => &$value) with key — verifies list type is preserved (already worked)
• foreach ($list as &$value) with typed list (list<string>) — verifies typed list is preserved
• foreach ($list as &$value) with arithmetic modification — verifies typed list is preserved

Fixes phpstan/phpstan#13809

Closes phpstan/phpstan#1311
Closes phpstan/phpstan#13851
Closes phpstan/phpstan#14083
Closes phpstan/phpstan#14084