Fix setting known offset

src/Analyser/MutatingScope.php

@@ -750,6 +750,13 @@ public function getType(Expr $node): Type
 			return $this->getType($node->getVar())->unsetOffset($this->getType($node->getDim()));
 		}
 		if ($node instanceof SetOffsetValueTypeExpr) {
+			$var = $node->getVar() instanceof OriginalPropertyTypeExpr ? $node->getVar()->getPropertyFetch() : $node->getVar();
+			if ($node->getDim() !== null && $this->hasExpressionType(new Expr\ArrayDimFetch($var, $node->getDim()))->yes()) {
+				return $this->getType($node->getVar())->setExistingOffsetValueType(
+					$this->getType($node->getDim()),
+					$this->getType($node->getValue()),
+				);
+			}
 			return $this->getType($node->getVar())->setOffsetValueType(
 				$node->getDim() !== null ? $this->getType($node->getDim()) : null,
 				$this->getType($node->getValue()),

tests/PHPStan/Rules/Arrays/data/offset-access-value-assignment.php

@@ -59,3 +59,20 @@ public function foo(): void
 		$this->collection2[] = 2;
 	}
 }
+
+class TestList
+{
+
+	/**
+	 * @var list<int>
+	 */
+	public array $list = [];
+
+	public function setKnownOffset(int $offset): void
+	{
+		if (isset($this->list[$offset])) {
+			$this->list[$offset] = 123;
+		}
+	}
+
+}