Fix tasty bootstrap problem

Author: odersky

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -190,12 +190,14 @@ class Erasure extends Phase with DenotTransformer {
   def assertErased(tp: Type, tree: tpd.Tree = tpd.EmptyTree)(using Context): Unit = {
     def isAllowed(cls: Symbol, sourceName: String) =
       tp.typeSymbol == cls && ctx.compilationUnit.source.file.name == sourceName
-    assert(isErasedType(tp) ||
-           isAllowed(defn.ArrayClass, "Array.scala") ||
-           isAllowed(defn.TupleClass, "Tuple.scala") ||
-           isAllowed(defn.NonEmptyTupleClass, "Tuple.scala") ||
-           isAllowed(defn.PairClass, "Tuple.scala"),
-        i"The type $tp - ${tp.toString} of class ${tp.getClass} of tree $tree : ${tree.tpe} / ${tree.getClass} is illegal after erasure, phase = ${ctx.phase.prev}")
+    assert(
+      isErasedType(tp)
+      || isAllowed(defn.ArrayClass, "Array.scala")
+      || isAllowed(defn.TupleClass, "Tuple.scala")
+      || isAllowed(defn.NonEmptyTupleClass, "Tuple.scala")
+      || isAllowed(defn.PairClass, "Tuple.scala")
+      || isAllowed(defn.PureClass, "Pure.scala"),
+      i"The type $tp - ${tp.toString} of class ${tp.getClass} of tree $tree : ${tree.tpe} / ${tree.getClass} is illegal after erasure, phase = ${ctx.phase.prev}")
   }
 }