On invalid prefix, throw a TypeError

Previously, the type was set to UnspecifiedErrorType. But that violates the rule
that every ErrorType has to be reported. If that does not happen, crashes in
pickler will result.

Author: odersky

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

@@ -2751,7 +2751,8 @@ object Types {
         NamedType(prefix, name, d)
       }
       if (prefix eq this.prefix) this
-      else if !NamedType.validPrefix(prefix) then UnspecifiedErrorType
+      else if !NamedType.validPrefix(prefix) then
+        throw TypeError(em"invalid new prefix $prefix cannot replace ${this.prefix} in type $this")
       else if (lastDenotation == null) NamedType(prefix, designator)
       else designator match {
         case sym: Symbol =>
@@ -5400,6 +5401,9 @@ object Types {
     def explanation(using Context): String = msg.message
   }
 
+  /** Note: Make sure an errors is reported before construtcing this
+   *  as the type of a tree.
+   */
   object ErrorType:
     def apply(m: Message)(using Context): ErrorType =
       val et = new PreviousErrorType

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

@@ -699,6 +699,9 @@ class TreePickler(pickler: TastyPickler) {
         case ex: AssertionError =>
           println(i"error when pickling tree $tree")
           throw ex
+       case ex: MatchError =>
+          println(i"error when pickling tree $tree")
+          throw ex
       }
   }
 

tests/neg/i18058.scala

@@ -0,0 +1,4 @@
+trait F:
+  type A
+
+type G = (f: _ <: F) => f.A // error