Fix #2064: Avoid illegal types in OrDominator

Need to skip type bounds in `underlying` chain, since
TypeBounds is not a legal operand type for OrType.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -239,7 +239,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
         case tp1: RecType =>
           tp1.rebind(approximateOr(tp1.parent, tp2))
         case tp1: TypeProxy if !isClassRef(tp1) =>
-          orDominator(tp1.superType | tp2)
+          orDominator(tp1.superType.bounds.hi | tp2)
         case _ =>
           tp2 match {
             case tp2: RecType =>

tests/pos/i2064.scala

@@ -0,0 +1,15 @@
+object p {
+
+class Foo[T] {
+  // Crashes:
+  def f(): Foo[T] = (if (true) this else this).bar
+
+  // Works:
+  // def f(): Foo[T] = new Bar(if (true) this else this).bar
+}
+
+implicit class Bar[A](val self: A) {
+  def bar(): A = self
+}
+
+}