From 2949e3da36cd3a6e691fbe262a513a37b69d1dcf Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
 <41898282+github-actions[bot]@users.noreply.github.com>
Date: Tue, 31 Mar 2026 03:48:41 +0000
Subject: [PATCH] Fix false positive "offset might not exist" for string offset
 in union

- Skip "might not exist" report for string types with integer offsets in
  NonexistentOffsetInArrayDimFetchCheck decomposition check
- When a union of constant strings (e.g. ''|':') is accessed with a valid
  integer offset, the decomposition incorrectly flagged it because one
  member (empty string) returned "no" for the offset
- The union-level check already correctly returns "maybe" for such cases
- New regression test in tests/PHPStan/Rules/Arrays/data/bug-13688.php

Closes https://github.com/phpstan/phpstan/issues/13688
---
 .../Arrays/NonexistentOffsetInArrayDimFetchCheck.php |  3 +++
 .../NonexistentOffsetInArrayDimFetchRuleTest.php     |  5 +++++
 tests/PHPStan/Rules/Arrays/data/bug-13688.php        | 12 ++++++++++++
 3 files changed, 20 insertions(+)
 create mode 100644 tests/PHPStan/Rules/Arrays/data/bug-13688.php

diff --git a/src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php b/src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php
index 263a62aebf9..08e0794d0ca 100644
--- a/src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php
+++ b/src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php
@@ -126,6 +126,9 @@ public function check(
 					if (
 						$innerType->hasOffsetValueType($innerDimType)->no()
 					) {
+						if ($innerType->isString()->yes() && $innerDimType->isInteger()->yes()) {
+							continue;
+						}
 						$report = true;
 						break 2;
 					}
diff --git a/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php b/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php
index 7def992a257..d61948406c7 100644
--- a/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php
+++ b/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php
@@ -1277,4 +1277,9 @@ public function testBug14308(): void
 		$this->analyse([__DIR__ . '/data/bug-14308.php'], []);
 	}
 
+	public function testBug13688(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13688.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Arrays/data/bug-13688.php b/tests/PHPStan/Rules/Arrays/data/bug-13688.php
new file mode 100644
index 00000000000..19a7d57a7af
--- /dev/null
+++ b/tests/PHPStan/Rules/Arrays/data/bug-13688.php
@@ -0,0 +1,12 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13688;
+
+$inputs = [ '', ':' ];
+
+foreach ( $inputs as $input )
+{
+	$inputLen = \strlen($input);
+	$hasTrailingColon = $inputLen > 0 && $input[$inputLen-1] === ':';
+	echo $hasTrailingColon ? "{$input} has trailing colon\n" : "{$input} does not have trailing colon\n";
+}