Do widening directly in isErroneous

Author: smarter

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

@@ -284,8 +284,12 @@ object Types {
     /** Is this type produced as a repair for an error? */
     final def isError(implicit ctx: Context): Boolean = stripTypeVar.isInstanceOf[ErrorType]
 
-    /** Is some part of this type produced as a repair for an error? */
-    def isErroneous(implicit ctx: Context): Boolean = existsPart(_.isError, forceLazy = false)
+    /** Is some part of this type (or of an underlying type of this type) produced as a repair for an error? */
+    def isErroneous(implicit ctx: Context): Boolean =
+      existsPart(_.isError, forceLazy = false) || {
+        val tpw = this.widen
+        (tpw `ne` this) && tpw.isErroneous
+      }
 
     /** Does the type carry an annotation that is an instance of `cls`? */
     @tailrec final def hasAnnotation(cls: ClassSymbol)(implicit ctx: Context): Boolean = stripTypeVar match {

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

@@ -2897,7 +2897,7 @@ class Typer extends Namer
   }
 
   private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol)(implicit ctx: Context): Unit = {
-    if (!tree.tpe.widen.isErroneous && !ctx.isAfterTyper && isPureExpr(tree) &&
+    if (!tree.tpe.isErroneous && !ctx.isAfterTyper && isPureExpr(tree) &&
         !tree.tpe.isRef(defn.UnitClass) && !isSelfOrSuperConstrCall(tree))
       ctx.warning(PureExpressionInStatementPosition(original, exprOwner), original.sourcePos)
   }