Avoid crash when superType does not exist after erasure

Fixes #19929

Two main changes:

 - In TypeErasure, throw a TypeError instead of a FatalError if a supertype of an applied
   type does not exist. That way, we get a proper error with a position.
 - Move some catch-and-rethrow logic from ReTyper to TreeChecker. ReTyper alreayd had special
   exceptions that disabled the logic for all uses of ReTyper except TreeChecker. Unfortunately
   the ReTyper override also disabled the special TypeError handling in Typer.

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

@@ -751,12 +751,12 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
   private def checkedSuperType(tp: TypeProxy)(using Context): Type =
     val tp1 = tp.translucentSuperType
     if !tp1.exists then
-      val msg = tp.typeConstructor match
+      val typeErr = tp.typeConstructor match
         case tycon: TypeRef =>
-          MissingType(tycon.prefix, tycon.name).toMessage.message
+          MissingType(tycon.prefix, tycon.name)
         case _ =>
-          i"Cannot resolve reference to $tp"
-      throw FatalError(msg)
+          TypeError(em"Cannot resolve reference to $tp")
+      throw typeErr
     tp1
 
   /** Widen term ref, skipping any `()` parameter of an eventual getter. Used to erase a TermRef.

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

@@ -46,7 +46,8 @@ abstract class TypeError(using creationContext: Context) extends Exception(""):
   def toMessage(using Context): Message
 
   /** Uses creationContext to produce the message */
-  override def getMessage: String = toMessage.message
+  override def getMessage: String =
+    try toMessage.message catch case ex: Throwable => "TypeError"
 
 object TypeError:
   def apply(msg: Message)(using Context) = new TypeError:

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

@@ -407,31 +407,35 @@ object TreeChecker {
     }
 
     override def typedUnadapted(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree = {
-      val res = tree match {
-        case _: untpd.TypedSplice | _: untpd.Thicket | _: EmptyValDef[?] =>
-          super.typedUnadapted(tree, pt, locked)
-        case _ if tree.isType =>
-          promote(tree)
-        case _ =>
-          val tree1 = super.typedUnadapted(tree, pt, locked)
-          def isSubType(tp1: Type, tp2: Type) =
-            (tp1 eq tp2) || // accept NoType / NoType
-            (tp1 <:< tp2)
-          def divergenceMsg(tp1: Type, tp2: Type) =
-            s"""Types differ
-               |Original type : ${tree.typeOpt.show}
-               |After checking: ${tree1.tpe.show}
-               |Original tree : ${tree.show}
-               |After checking: ${tree1.show}
-               |Why different :
-             """.stripMargin + core.TypeComparer.explained(_.isSubType(tp1, tp2))
-          if (tree.hasType) // it might not be typed because Typer sometimes constructs new untyped trees and resubmits them to typedUnadapted
-            assert(isSubType(tree1.tpe, tree.typeOpt), divergenceMsg(tree1.tpe, tree.typeOpt))
-          tree1
-      }
-      checkNoOrphans(res.tpe)
-      phasesToCheck.foreach(_.checkPostCondition(res))
-      res
+      try
+        val res = tree match
+          case _: untpd.TypedSplice | _: untpd.Thicket | _: EmptyValDef[?] =>
+            super.typedUnadapted(tree, pt, locked)
+          case _ if tree.isType =>
+            promote(tree)
+          case _ =>
+            val tree1 = super.typedUnadapted(tree, pt, locked)
+            def isSubType(tp1: Type, tp2: Type) =
+              (tp1 eq tp2) || // accept NoType / NoType
+              (tp1 <:< tp2)
+            def divergenceMsg(tp1: Type, tp2: Type) =
+              s"""Types differ
+                |Original type : ${tree.typeOpt.show}
+                |After checking: ${tree1.tpe.show}
+                |Original tree : ${tree.show}
+                |After checking: ${tree1.show}
+                |Why different :
+              """.stripMargin + core.TypeComparer.explained(_.isSubType(tp1, tp2))
+            if (tree.hasType) // it might not be typed because Typer sometimes constructs new untyped trees and resubmits them to typedUnadapted
+              assert(isSubType(tree1.tpe, tree.typeOpt), divergenceMsg(tree1.tpe, tree.typeOpt))
+            tree1
+        checkNoOrphans(res.tpe)
+        phasesToCheck.foreach(_.checkPostCondition(res))
+        res
+      catch case NonFatal(ex) if !ctx.run.enrichedErrorMessage =>
+        val treeStr = tree.show(using ctx.withPhase(ctx.phase.prev.megaPhase))
+        println(ctx.run.enrichErrorMessage(s"exception while retyping $treeStr of class ${tree.className} # ${tree.uniqueId}"))
+        throw ex
     }
 
     def checkNotRepeated(tree: Tree)(using Context): tree.type = {

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

@@ -170,13 +170,6 @@ class ReTyper(nestingLevel: Int = 0) extends Typer(nestingLevel) with ReChecking
   override def addCanThrowCapabilities(expr: untpd.Tree, cases: List[CaseDef])(using Context): untpd.Tree =
     expr
 
-  override def typedUnadapted(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree =
-    try super.typedUnadapted(tree, pt, locked)
-    catch case NonFatal(ex) if ctx.phase != Phases.typerPhase && ctx.phase != Phases.inliningPhase && !ctx.run.enrichedErrorMessage =>
-      val treeStr = tree.show(using ctx.withPhase(ctx.phase.prev.megaPhase))
-      println(ctx.run.enrichErrorMessage(s"exception while retyping $treeStr of class ${tree.className} # ${tree.uniqueId}"))
-      throw ex
-
   override def inlineExpansion(mdef: DefDef)(using Context): List[Tree] = mdef :: Nil
 
   override def inferView(from: Tree, to: Type)(using Context): Implicits.SearchResult =

tests/neg/i19929.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/i19929.scala:5:6 -----------------------------------------------------------------------------------
+5 |  val _: a.M = ??? // error was crash
+  |      ^
+  |      cannot resolve reference to type (a : A).M
+  |      the classfile defining the type might be missing from the classpath

tests/neg/i19929.scala

@@ -0,0 +1,5 @@
+trait A:
+  private type M
+
+def foo(a: A{type M = Int}) =
+  val _: a.M = ??? // error was crash