Narrow types after strlen()

src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php

@@ -126,6 +126,9 @@
 					if (
 						$innerType->hasOffsetValueType($innerDimType)->no()
 					) {
+						if ($innerType->isString()->yes() && $innerDimType->isInteger()->yes()) {
+							continue;
+						}
 						$report = true;
 						break 2;
 					}

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1343,4 +1343,9 @@ public function testArrayFindKeyExisting(): void
 		]);
 	}
 
+	public function testBug13688(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13688.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-13688.php

@@ -0,0 +1,28 @@
+<?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";
+}
+
+/** @var 'a'|'abc' $str */
+$str = 'a';
+echo $str[0];
+
+/** @var 'a'|'abc' $str2 */
+$str2 = 'a';
+echo $str2[2];
+
+/** @var ''|'foo'|'barbaz' $str3 */
+$str3 = 'foo';
+echo $str3[4];
+
+/** @var string|array<int, string> $mixed */
+$mixed = 'test';
+echo $mixed[0];