Give reasonable error messages when classfiles are not found

Fixes #6501
Fixes #8405

Both tests exercise corner cases where a type seems to be present at first
but then is missing. It took some effort to dig out the useful error message
from excetiopn handling code.

Author: odersky

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

@@ -832,9 +832,10 @@ object Denotations {
 
     private def updateValidity()(implicit ctx: Context): this.type = {
       assert(
-        ctx.runId >= validFor.runId ||
-        ctx.settings.YtestPickler.value || // mixing test pickler with debug printing can travel back in time
-        symbol.is(Permanent),              // Permanent symbols are valid in all runIds
+        ctx.runId >= validFor.runId
+        || ctx.settings.YtestPickler.value // mixing test pickler with debug printing can travel back in time
+        || ctx.mode.is(Mode.Printing)  // no use to be picky when printing error messages
+        || symbol.isOneOf(ValidForeverFlags),
         s"denotation $this invalid in run ${ctx.runId}. ValidFor: $validFor")
       var d: SingleDenotation = this
       while ({

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

@@ -17,12 +17,14 @@ import config.Printers.cyclicErrors
 
 class TypeError(msg: String) extends Exception(msg) {
   def this() = this("")
-  def toMessage(implicit ctx: Context): Message = super.getMessage
+  final def toMessage(implicit ctx: Context): Message =
+    produceMessage(using ctx.addMode(Mode.Printing))
+  def produceMessage(using Context): Message = super.getMessage
   override def getMessage: String = super.getMessage
 }
 
 class MalformedType(pre: Type, denot: Denotation, absMembers: Set[Name]) extends TypeError {
-  override def toMessage(implicit ctx: Context): Message =
+  override def produceMessage(implicit ctx: Context): Message =
     i"malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}"
 }
 
@@ -33,7 +35,7 @@ class MissingType(pre: Type, name: Name) extends TypeError {
     case _ => ""
   }
 
-  override def toMessage(implicit ctx: Context): Message = {
+  override def produceMessage(implicit ctx: Context): Message = {
     if (ctx.debug) printStackTrace()
     i"""cannot resolve reference to type $pre.$name
        |the classfile defining the type might be missing from the classpath${otherReason(pre)}"""
@@ -67,7 +69,7 @@ class RecursionOverflow(val op: String, details: => String, val previous: Throwa
       (rs.map(_.explanation): List[String]).mkString("\n  ", "\n|  ", "")
   }
 
-  override def toMessage(implicit ctx: Context): Message = {
+  override def produceMessage(implicit ctx: Context): Message = {
     val mostCommon = recursions.groupBy(_.op).toList.maxBy(_._2.map(_.weight).sum)._2.reverse
     s"""Recursion limit exceeded.
        |Maybe there is an illegal cyclic reference?
@@ -109,7 +111,7 @@ object handleRecursive {
 class CyclicReference private (val denot: SymDenotation) extends TypeError {
   var inImplicitSearch: Boolean = false
 
-  override def toMessage(implicit ctx: Context): Message = {
+  override def produceMessage(implicit ctx: Context): Message = {
     val cycleSym = denot.symbol
 
     // cycleSym.flags would try completing denot and would fail, but here we can use flagsUNSAFE to detect flags
@@ -182,7 +184,7 @@ class MergeError(val sym1: Symbol, val sym2: Symbol, val tp1: Type, val tp2: Typ
       s"\nas members of $owner"
     }
 
-  override def toMessage(implicit ctx: Context): Message = {
+  override def produceMessage(implicit ctx: Context): Message = {
     if (ctx.debug) printStackTrace()
     i"""cannot merge
        |  ${showSymbol(sym1)} of type ${showType(tp1)}  and

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -606,6 +606,8 @@ class TreePickler(pickler: TastyPickler) {
           }
       }
       catch {
+        case ex: TypeError =>
+          ctx.error(ex.toMessage, tree.sourcePos.focus)
         case ex: AssertionError =>
           println(i"error when pickling tree $tree")
           throw ex

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

@@ -124,7 +124,7 @@ class NoExplanation(val msg: String) extends Message(ErrorMessageID.NoExplanatio
   val explanation: String = ""
   val kind: String = ""
 
-  override def toString(): String = s"NoExplanation($msg)"
+  override def toString(): String = msg
 }
 
 /** The extractor for `NoExplanation` can be used to check whether any error

tests/neg/i6501.scala

@@ -0,0 +1,24 @@
+import scala.collection.immutable.HashMap
+
+trait MapImpl {
+  type Key
+  type Value
+  type Map
+  val lookup: Map => Key => Value
+}
+class HashMapImpl[K, V] extends MapImpl {
+  type Key = K
+  type Value = V
+  type Map = HashMap[K, V]
+  val lookup: Map => Key => Value = m => k => m(k)
+}
+object Foo {
+  val Server0:
+    (mImpl: MapImpl) => mImpl.Map => mImpl.Key => mImpl.Value
+    = mImpl => mImpl.lookup
+  val Client:
+    (server: (mImpl: MapImpl & {type Key = String} & {type Value = Int}) => mImpl.Map => String => Int) => Int =
+    // server => server(??? : (HashMapImpl[String, Int]))(???)("test lookup key") //works
+    // server => server(HashMapImpl[String, Int])(???)("") //works
+    server => server(???)(???)("test lookup key") // error
+}

tests/neg/i8405.scala

@@ -0,0 +1,11 @@
+class ActorRef
+trait ActorEventBus {
+  type Subscriber = ActorRef
+}
+trait ManagedActorClassification { this: ActorEventBus =>
+  def unsubscribe(subscriber: Subscriber): Unit = ???
+}
+class ActorClassificationUnsubscriber(bus: ManagedActorClassification) {
+  val actor = ???
+  bus.unsubscribe(actor)  // error
+}