Check that union types do not contain singleton types.

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

@@ -403,6 +403,11 @@ trait Checking {
       else tpt.withType(alias)
     }
     else tpt
+
+  /** Check that `tpt` does not define a singleton type */
+  def checkNoSingleton(tpt: Tree)(implicit ctx: Context): Tree =
+    if (tpt.tpe.isInstanceOf[SingletonType]) errorTree(tpt, i"no singleton type allowed here")
+    else tpt
 }
 
 trait NoChecking extends Checking {
@@ -417,4 +422,5 @@ 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 checkNoSingleton(tpt: Tree)(implicit ctx: Context): Tree = tpt
 }

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

@@ -800,8 +800,8 @@ 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 left1 = checkNoSingleton(typed(tree.left))
+    val right1 = checkNoSingleton(typed(tree.right))
     assignType(cpy.OrTypeTree(tree)(left1, right1), left1, right1)
   }
 

test/dotc/tests.scala

@@ -108,6 +108,7 @@ class tests extends CompilerTest {
   @Test def neg_typers() = compileFile(negDir, "typers", xerrors = 14)(allowDoubleBindings)
   @Test def neg_privates() = compileFile(negDir, "privates", xerrors = 2)
   @Test def neg_rootImports = compileFile(negDir, "rootImplicits", xerrors = 2)
+  @Test def neg_singletonsLubs = compileFile(negDir, "singletons-lubs", xerrors = 6)
   @Test def neg_templateParents() = compileFile(negDir, "templateParents", xerrors = 3)
   @Test def neg_autoTupling = compileFile(posDir, "autoTuplingTest", args = "-language:noAutoTupling" :: Nil, xerrors = 3)
   @Test def neg_autoTupling2 = compileFile(negDir, "autoTuplingTest", xerrors = 3)

tests/neg/singletons-lubs.scala

@@ -0,0 +1,7 @@
+object Test {
+  def oneOrTwo(x: 1 | 2): 1 | 2 = x // error // error // error // error
+  def test: Unit = {
+    val foo: 3 | 4 = 1 // error // error
+    oneOrTwo(foo)
+  }
+}