Add evaluation tests for multiple assignments

Author: kyouko-taiga

tests/run/multiple-assignment.check

@@ -0,0 +1,12 @@
+after swap: (2, 4)
+after swap: (2, 4)
+after swap: (2, 4)
+load 1
+load 2
+load 2
+load 4 from 2
+load 1
+load 2 from 1
+store 4 to 1
+store 2 to 2
+after swap: (4, 2)

tests/run/multiple-assignment.scala

@@ -0,0 +1,38 @@
+object Test:
+
+  class Container[T](val id: Int, var x: T):
+
+    def y: T =
+      println(s"load ${x} from ${id}")
+      x
+
+    def y_=(newValue: T): Unit =
+      println(s"store ${newValue} to ${id}")
+      this.x_=(newValue)
+
+  def main(args: Array[String]): Unit =
+    // simple swap
+    var x1 = 4
+    var x2 = 2
+    (x1, x2) = (x2, x1)
+    println(s"after swap: (${x1}, ${x2})")
+
+    // swap in a container
+    val a = Array(4, 2)
+    (a(0), a(1)) = (a(1), a(0))
+    println(s"after swap: (${a(0)}, ${a(1)})")
+
+    // swap fields with effectless left-hand sides
+    var c1 = Container(1, 4)
+    var c2 = Container(2, 2)
+    (c1.x, c2.x) = (c2.x, c1.x)
+    println(s"after swap: (${c1.x}, ${c2.x})")
+
+    // swap fields with side effectful left-hand sides
+    def f(n: Int): Container[Int] =
+      println(s"load ${n}")
+      n match
+        case 1 => c1
+        case 2 => c2
+    (f(1).y, f(2).y) = (f(2).y, f(1).y)
+    println(s"after swap: (${c1.x}, ${c2.x})")