Fix union with error

Author: smarter

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

@@ -185,12 +185,16 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
           tp1.rebind(approximateOr(tp1.parent, tp2))
         case tp1: TypeProxy if !isClassRef(tp1) =>
           orDominator(tp1.superType | tp2)
+        case err: ErrorType =>
+          err
         case _ =>
           tp2 match {
             case tp2: RecType =>
               tp2.rebind(approximateOr(tp1, tp2.parent))
             case tp2: TypeProxy if !isClassRef(tp2) =>
               orDominator(tp1 | tp2.superType)
+            case err: ErrorType =>
+              err
             case _ =>
               val commonBaseClasses = tp.mapReduceOr(_.baseClasses)(intersect)
               val doms = dominators(commonBaseClasses, Nil)

tests/neg/errorTypes.scala

@@ -2,4 +2,10 @@ object Test {
   val x: iDontExist = 1 // error: not found: type iDontExist
 
   val y = x.asInstanceOf[Int] // No error reported (was: value asInstanceOf does not take type parameters)
+
+  val a: iDontExist | Int = 1 // error: not found: type iDontExist
+  val a2 = a.isInstanceOf[Int] // No error (used to crash)
+
+  val b: iDontExist & Int = 1 // error: not found: type iDontExist
+  val b2 = a.isInstanceOf[Int] // No error (worked before too)
 }