Disallow singleton types in unions

For the moment, we do not know how to handle something like

    1 | 2

or

    x.type | y.type

correctly. So it's better to disallow these situations until we find a proper
solution.

Author: odersky

src/dotty/tools/dotc/typer/Checking.scala

@@ -542,6 +542,13 @@ trait Checking {
       errorTree(tpt, ex"missing type parameter for ${tpt.tpe}")
     }
     else tpt
+
+  /** Check that `tpt` does not refer to a singleton type */
+  def checkNotSingleton(tpt: Tree, where: String)(implicit ctx: Context): Tree =
+    if (tpt.tpe.isInstanceOf[SingletonType]) {
+      errorTree(tpt, ex"Singleton type ${tpt.tpe} is not allowed $where")
+    }
+    else tpt
 }
 
 trait NoChecking extends Checking {
@@ -556,4 +563,5 @@ trait NoChecking extends Checking {
   override def checkNoDoubleDefs(cls: Symbol)(implicit ctx: Context): Unit = ()
   override def checkParentCall(call: Tree, caller: ClassSymbol)(implicit ctx: Context) = ()
   override def checkSimpleKinded(tpt: Tree)(implicit ctx: Context): Tree = tpt
+  override def checkNotSingleton(tpt: Tree, where: String)(implicit ctx: Context): Tree = tpt
 }

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

@@ -994,8 +994,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   }
 
   def typedOrTypeTree(tree: untpd.OrTypeTree)(implicit ctx: Context): OrTypeTree = track("typedOrTypeTree") {
-    val left1 = typed(tree.left)
-    val right1 = typed(tree.right)
+    val where = "in a union type"
+    val left1 = checkNotSingleton(typed(tree.left), where)
+    val right1 = checkNotSingleton(typed(tree.right), where)
     assignType(cpy.OrTypeTree(tree)(left1, right1), left1, right1)
   }
 

tests/neg/singletonOrs.scala

@@ -0,0 +1,6 @@
+object Test {
+   def foo: 1 | 2 = 1 // error // error
+   def bar: 3 | 4 = foo // error // error
+   def foo: 1 | 2 = 1 // error // error
+   def bar: 1 = foo
+}