Survive inaccessible types when computing implicit scope

Also: Give a better error message later when encountering a missing type that
refers to a private member of a base class. The previous one was misleading since
it referred to a potentially missing class file, which is certainly not the case here.

Fixes #21543

[Cherry-picked 472555d][modified]

Author: WojciechMazur

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

@@ -73,6 +73,8 @@ class MissingType(val pre: Type, val name: Name)(using Context) extends TypeErro
       case _ if givenSelf.exists && givenSelf.member(name).exists =>
         i"""$name exists as a member of the self type $givenSelf of $cls
            |but it cannot be called on a receiver whose type does not extend $cls"""
+      case _ if pre.baseClasses.exists(_.findMember(name, pre, Private, EmptyFlags).exists) =>
+        i"$name is a private member in a base class"
       case _ =>
         missingClassFile
 

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

@@ -781,6 +781,10 @@ trait ImplicitRunInfo:
           override def stopAt = StopAt.Static
           private val seen = util.HashSet[Type]()
 
+          override def derivedTypeBounds(tp: TypeBounds, lo: Type, hi: Type): Type =
+            if lo.exists && hi.exists then super.derivedTypeBounds(tp, lo, hi)
+            else NoType // Survive inaccessible types, for instance in i21543.scala.
+
           def applyToUnderlying(t: TypeProxy) =
             if seen.contains(t) then
               WildcardType

tests/neg/i21543.scala

@@ -0,0 +1,13 @@
+object CompilerCrash {
+  trait Scope {
+    private type Event = String
+
+    case class Cmd(events: List[Event])
+  }
+
+  new Scope {
+    val commands = List(
+      Cmd(List("1", "2"))
+    )
+  }
+}