Skip "might not exist" report for string types with integer offsets in union decomposition check

- In NonexistentOffsetInArrayDimFetchCheck, the reportMaybes decomposition
  loop flattens union types and checks each member individually. When a
  constant string member (e.g. '') returns "no" for an integer offset (e.g. 0),
  it incorrectly triggers "might not exist" even though the union-level check
  correctly returns "maybe".
- Skip the report when innerType is a string and innerDimType is an integer,
  since string offset validity is length-dependent and users typically guard
  with length checks through variables that PHPStan cannot track.
- The fix does not affect: definite "does not exist" errors (caught earlier),
  non-integer offsets on strings (objects, floats, strings still report), or
  array offset checks (completely unaffected).
- Probed analogous cases: non-integer dim types on strings, array offset
  decomposition, ArrayDestructuringRule (shares the same check) — all
  already correct.

Author: phpstan-bot

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;
 					}

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1326,6 +1326,11 @@ public function testBug9240(): void
 		$this->analyse([__DIR__ . '/data/bug-9240.php'], []);
 	}
 
+	public function testBug13688(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13688.php'], []);
+	}
+
 	#[RequiresPhp('>= 8.4.0')]
 	public function testArrayFindKeyExisting(): void
 	{

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

@@ -0,0 +1,19 @@
+<?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";
+}
+
+function directComparison(): void
+{
+	/** @var 'a'|'abc' $s */
+	$s = 'a';
+	echo $s[0];
+}