Fix phpstan/phpstan#12163: non-negative-int evaluated to int

[phpstan-bot]

Summary

When a variable was assigned 0 and then modified inside a for loop (incremented or reset to 0), PHPStan incorrectly generalized its type to int instead of preserving int<0, max> (non-negative-int). This caused false positive errors like "expects int<0, max>, int given".

Changes

• Modified MutatingScope::generalizeType() in src/Analyser/MutatingScope.php
• Changed the constant integers generalization logic for the case when values expand in both directions (gotGreater && gotSmaller)
• Instead of immediately widening to IntegerType (plain int), the fix computes the actual observed min/max bounds from both iterations using IntegerRangeType::fromInterval($newMin, $newMax)
• Added regression test in tests/PHPStan/Analyser/nsrt/bug-12163.php

Root cause

In generalizeType(), constant integer values from consecutive loop iterations are compared. If values in the current iteration are both greater AND smaller than the previous iteration's range, the code assumed both bounds were unstable and widened to int.

In the reported case, $columnIndex started at 0, was incremented ($columnIndex++) in one branch and reset to 0 in another. After the first loop iteration, the type was 1 (only the increment path was reachable). After the second iteration (merged with init scope), it became 0|1|2. The generalization saw 1 → 0|1|2 — values grew in both directions — and concluded int.

The fix computes the actual combined bounds (int<0, 2>) instead. On the next generalization iteration, this becomes an IntegerRangeType, which the integer range generalization logic handles correctly — it sees only upward growth and produces int<0, max>. For truly unbounded cases (both directions growing indefinitely), the integer range generalization in the subsequent iteration still correctly produces int.

Test

Added tests/PHPStan/Analyser/nsrt/bug-12163.php which reproduces the original issue: a for loop with two index variables where one is incremented or reset to 0. Both $rowIndex and $columnIndex are asserted to be int<0, max>.

Fixes phpstan/phpstan#12163

[staabm]

@phpstan-bot the fix and test applies to lines 4115-4127 in MutatingScope. for symmetry we need the same fix, covered by a test, in line 4207 of MutatingScope