Fix phpstan/phpstan#13857: Incorrect narrowing of nested array after assignment

[phpstan-bot]

Summary

Fixes phpstan/phpstan#13857

When assigning to a nested array with a non-constant key ($array[$id]['state'] = 'foo'), PHPStan incorrectly narrowed the type of ALL elements in the outer array, not just the element being modified. This caused array<int, array{state: string}> to become non-empty-array<int, array{state: 'foo'}>.

Root cause

In AssignHandler::produceArrayDimFetchAssignValueToWrite(), the reverse loop that processes nested array dim fetch assignments from innermost to outermost passed $unionValues = ($i === 0) to setOffsetValueType(). For outer levels ($i > 0) with non-constant keys, this caused ArrayType::setOffsetValueType() to replace the entire item type with the new value type, discarding the original item type.

Fix

The fix changes the $unionValues logic so that non-constant keys (keys with no constant scalar decomposition) always use union semantics, preserving the original item type alongside the new value type:

$unionValues = $i === 0
    || ($offsetType !== null && count($offsetType->getConstantScalarTypes()) === 0);

This correctly reflects that when modifying $array[$id]['state'] where $id is non-constant, we only know that ONE element was modified — the other elements retain their original types.

Test expectation changes

Several existing test expectations were updated to reflect the more correct (but less precise) types:

• assign-nested-arrays.php: {bar: 1, baz: 2} → {bar: 1, baz?: 2} (second assignment to same non-constant key can't guarantee both keys exist on all elements)
• pr-4390.php: Inner array key range lost (non-constant loop variable key)
• bug-10438.php: list<string> → list<string>|string (union with existing item type)
• bug-11679.php: array{foo: true} → array{foo?: bool} (only one element modified)
• bug-12927.php: Lost hasOffsetValue precision for non-constant key assignment
• slevomat-foreach-array-key-exists-bug.php: More general item type after non-constant key modification
• AutowiredAttributeServicesExtension.php: PHPDoc changed non-empty-list to list (append in loop with non-constant key)

All updated types are type-theoretically correct — the previous types were over-precise, incorrectly claiming knowledge about all array elements when only one was modified.