Fix ConstantArrayType::hasOffsetValueType() for offset union type whe…

…re all types are valid

src/Type/Constant/ConstantArrayType.php

@@ -610,6 +610,14 @@ public function findTypeAndMethodNames(): array
 	public function hasOffsetValueType(Type $offsetType): TrinaryLogic
 	{
 		$offsetType = $offsetType->toArrayKey();
+		if ($offsetType instanceof UnionType) {
+			$results = [];
+			foreach ($offsetType->getTypes() as $innerType) {
+				$results[] = $this->hasOffsetValueType($innerType);
+			}
+
+			return TrinaryLogic::extremeIdentity(...$results);
+		}
 
 		$result = TrinaryLogic::createNo();
 		foreach ($this->keyTypes as $i => $keyType) {

tests/PHPStan/Rules/Arrays/data/report-possibly-nonexistent-array-offset.php

@@ -39,4 +39,14 @@ public function testConstantArray2(array $a): void
 			echo $a[0];
 		}
 	}
+
+	/**
+	 * @param array{0: '9', A: 'Z', a: 'z'} $a
+	 * @param '0'|'A'|'a' $dim
+	 */
+	public function testDimUnion(array $a, string $dim): void
+	{
+		echo $a[$dim];
+	}
+
 }