Add fix

Author: VincentLanglet

src/Rules/Arrays/NonexistentOffsetInArrayDimFetchCheck.php

@@ -12,6 +12,7 @@
 use PHPStan\Rules\RuleLevelHelper;
 use PHPStan\Type\BenevolentUnionType;
 use PHPStan\Type\ErrorType;
+use PHPStan\Type\MixedType;
 use PHPStan\Type\NeverType;
 use PHPStan\Type\Type;
 use PHPStan\Type\TypeCombinator;
@@ -86,7 +87,9 @@ public function check(
 				$flattenedTypes = TypeUtils::flattenTypes($type);
 			}
 
-			$validArrayDimType = TypeCombinator::intersect(AllowedArrayKeysTypes::getType(), $dimType);
+			$validArrayDimType = $dimType instanceof MixedType
+				? $dimType
+				: TypeCombinator::intersect(AllowedArrayKeysTypes::getType(), $dimType);
 
 			foreach ($flattenedTypes as $innerType) {
 				$dimTypeToCheck = $innerType->isArray()->yes() ? $validArrayDimType : $dimType;

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1094,6 +1094,11 @@ public function testPR4385(): void
 		]);
 	}
 
+	public function testPR4385Bis(): void
+	{
+		$this->analyse([__DIR__ . '/data/pr-4385-bis.php'], []);
+	}
+
 	public function testBug12805(): void
 	{
 		$this->reportPossiblyNonexistentGeneralArrayOffset = true;

tests/PHPStan/Rules/Arrays/data/pr-4385-bis.php

@@ -0,0 +1,12 @@
+<?php declare(strict_types = 1);
+
+namespace Pr4385Bis;
+
+class HelloWorld
+{
+	/** @param array<string, int> $a */
+	public function sayHello(array $a, mixed $m): void
+	{
+		echo $a[$m];
+	}
+}