Fix #3161: Check that operands of &, | are simple-kinded

Author: odersky

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1077,15 +1077,15 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   }
 
   def typedAndTypeTree(tree: untpd.AndTypeTree)(implicit ctx: Context): AndTypeTree = track("typedAndTypeTree") {
-    val left1 = typed(tree.left)
-    val right1 = typed(tree.right)
+    val left1 = checkSimpleKinded(typed(tree.left))
+    val right1 = checkSimpleKinded(typed(tree.right))
     assignType(cpy.AndTypeTree(tree)(left1, right1), left1, right1)
   }
 
   def typedOrTypeTree(tree: untpd.OrTypeTree)(implicit ctx: Context): OrTypeTree = track("typedOrTypeTree") {
     val where = "in a union type"
-    val left1 = checkNotSingleton(typed(tree.left), where)
-    val right1 = checkNotSingleton(typed(tree.right), where)
+    val left1 = checkNotSingleton(checkSimpleKinded(typed(tree.left)), where)
+    val right1 = checkNotSingleton(checkSimpleKinded(typed(tree.right)), where)
     assignType(cpy.OrTypeTree(tree)(left1, right1), left1, right1)
   }
 

tests/neg/i3161.scala

@@ -0,0 +1,10 @@
+object UnionTypes {
+  trait List[A]
+  class Empty() extends List[Nothing]
+  class Cons[A](h: A, t: List[A]) extends List[A]
+
+  def test: Unit = {
+    val list: Cons | Empty = null // error: missing type parameter
+    val list2: Empty & Cons = ??? // error: missing type parameter
+  }
+}