Fix phpstan/phpstan#13688: Offset might not exist regression from phpstan 2.1.23

[phpstan-bot]

Summary

False positive "Offset 0 might not exist on ''|':'" when accessing a string offset guarded by a strlen check through an intermediate variable ($inputLen = strlen($input); $inputLen > 0 && $input[$inputLen-1]).

Changes

• Modified src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php: In the reportMaybes decomposition loop, skip reporting when the inner type is a string and the offset is an integer. This prevents the aggressive union decomposition from flagging valid string offset access patterns.
• Added regression test tests/PHPStan/Rules/Arrays/data/bug-13688.php
• Added test method testBug13688 in tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

Root cause

The NonexistentOffsetInArrayDimFetchCheck rule decomposes union types and checks each member individually for offset existence. For a union like ''|':' with offset 0, the empty string '' correctly returns "no" for hasOffsetValueType(0), while ':' returns "yes". The union-level check returns "maybe", which is correct. However, the decomposition loop then flags the error because it finds one member that returns "no".

For strings, this decomposition is too aggressive because string offset validity is tied to string length. When the user has already guarded the access with a length check (e.g., $inputLen > 0), PHPStan narrows the length variable but can't propagate that narrowing back to the string type through the intermediate variable assignment. The fix skips the "might not exist" report for string types with integer offsets, relying on the union-level "maybe" check which is sufficient.

Test

Added tests/PHPStan/Rules/Arrays/data/bug-13688.php which reproduces the exact scenario from the issue: iterating over ['', ':'], checking strlen > 0 before accessing string offset. The test expects no errors.

Fixes phpstan/phpstan#13688