Fix phpstan/phpstan#14265: Invalid "Offset might not exist on array"

[phpstan-bot]

Summary

When using array_keys($a) as input to a higher-order function like array_reduce, accessing $a[$key] inside the callback was incorrectly reported as "Offset might not exist" for arrays with optional keys. The keys returned by array_keys() are guaranteed to exist in the source array, so this was a false positive.

Changes

• Added detectArrayKeysInSiblingArgs() method in src/Analyser/NodeScopeResolver.php that detects when array_keys($expr) appears as a sibling argument to a closure/arrow function in a function call
• Added applyArrayKeysSourceToScope() method that injects type information for $expr[$param] into the callback scope, matching callback parameters whose type equals the source array's key type
• Applied the detection in both the closure and arrow function processing paths within processArgs()
• Applied the scope injection in both processArrowFunctionNode() and processClosureNode()
• New regression test in tests/PHPStan/Rules/Arrays/data/bug-14265.php
• New test method testBug14265() in NonexistentOffsetInArrayDimFetchRuleTest.php

Root cause

PHPStan already had special handling for foreach (array_keys($a) as $key) that assigned the type of $a[$key] in the loop scope, recognizing that keys from array_keys() are always valid offsets. However, this handling didn't extend to callbacks passed to higher-order functions like array_reduce(array_keys($a), fn($carry, $key) => $a[$key], ''). The type system lost the correlation between the array_keys() result and the source array, so $a[$key] was checked independently, producing a false "might not exist" warning for optional keys.

The fix extends the same approach: when a sibling argument in a function call is array_keys($expr), the callback scope is enriched with the knowledge that $expr[$param] has a valid type, preventing the false positive.

Test

Added a regression test reproducing the exact scenario from the issue: an array with an optional key (k2?), array_keys() passed to array_reduce, and the callback accessing the array with the key parameter. The test expects no errors.

Fixes phpstan/phpstan#14265