Fix #5791: let t.asInstanceOf[Unit] always succeed

Author: abgruszecki

compiler/src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -234,6 +234,10 @@ object TypeTestsCasts {
           def testCls = testType.widen.classSymbol
           if (expr.tpe <:< testType)
             Typed(expr, tree.args.head)
+          else if (testCls eq defn.BoxedUnitClass) {
+            // as a special case, casting to Unit always successfully returns Unit
+            Block(expr :: Nil, Literal(Constant(()))).withSpan(expr.span)
+          }
           else if (foundCls.isPrimitiveValueClass) {
             if (testCls.isPrimitiveValueClass) primitiveConversion(expr, testCls)
             else derivedTree(box(expr), defn.Any_asInstanceOf, testType)

tests/run/i5791.check

@@ -0,0 +1 @@
+hello!

tests/run/i5791.scala

@@ -0,0 +1,9 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val a = { 5 }.asInstanceOf[Unit]
+    val b = {
+      println("hello!")
+      5
+    }.asInstanceOf[Unit]
+  }
+}