Avoid creating cyclic types when reporting errors in selectionType

The previous code was a great demonstration of the perils of mixing mutable variables with
delayed evaluation.

Also, make lazy vals in Message @ThreadUnsafe. This means that any recursion (like the one we
encountered before the fix) manifests itself in stackoverflows instead of deadlocks.

Fixes #12220

Author: odersky

compiler/src/dotty/tools/dotc/reporting/Message.scala

@@ -1,7 +1,7 @@
 package dotty.tools
 package dotc
 package reporting
-
+import scala.annotation.threadUnsafe
 import util.SourcePosition
 
 object Message {
@@ -85,10 +85,10 @@ abstract class Message(val errorId: ErrorMessageID) { self =>
   def rawMessage = message
 
   /** The message to report. <nonsensical> tags are filtered out */
-  lazy val message: String = dropNonSensical(msg + msgSuffix)
+  @threadUnsafe lazy val message: String = dropNonSensical(msg + msgSuffix)
 
   /** The explanation to report. <nonsensical> tags are filtered out */
-  lazy val explanation: String = dropNonSensical(explain)
+  @threadUnsafe lazy val explanation: String = dropNonSensical(explain)
 
   /** A message is non-sensical if it contains references to <nonsensical>
    *  tags.  Such tags are inserted by the error diagnostic framework if a

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

@@ -118,11 +118,13 @@ trait TypeAssigner {
   /** The type of the selection `tree`, where `qual1` is the typed qualifier part. */
   def selectionType(tree: untpd.RefTree, qual1: Tree)(using Context): Type =
     var qualType = qual1.tpe.widenIfUnstable
-    if !qualType.hasSimpleKind && tree.name != nme.CONSTRUCTOR then
-      // constructors are selected on typeconstructor, type arguments are passed afterwards
-      qualType = errorType(em"$qualType takes type parameters", qual1.srcPos)
-    else if !qualType.isInstanceOf[TermType] then
-      qualType = errorType(em"$qualType is illegal as a selection prefix", qual1.srcPos)
+    if !qualType.isError then
+      val prevQual = qualType
+      if !qualType.hasSimpleKind && tree.name != nme.CONSTRUCTOR then
+        // constructors are selected on typeconstructor, type arguments are passed afterwards
+        qualType = errorType(em"$prevQual takes type parameters", qual1.srcPos)
+      else if !qualType.isInstanceOf[TermType] then
+        qualType = errorType(em"$prevQual is illegal as a selection prefix", qual1.srcPos)
 
     def arrayElemType = qual1.tpe.widen match
       case JavaArrayType(elemtp) => elemtp

tests/neg/i12220.scala

@@ -0,0 +1,3 @@
+val a: List[Any] = List(List(1,2), List(3,4))
+val _ = for(b <- a ; c <- b.asInstanceOf[List]) { println(c) } // error
+