On invalid prefix, throw a TypeError

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/i12448.scala

@@ -1,5 +1,5 @@
 object Main {
   def mkArray[T <: A]: T#AType // error // error
-  mkArray[Array] // was: "assertion failed: invalid prefix HKTypeLambda..."
-  val x = mkArray[Array]
+  mkArray[Array] // was: "assertion failed: invalid prefix HKTypeLambda..." // error
+  val x = mkArray[Array] // error
 }

tests/neg/i16842.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i16842.scala:24:7 ----------------------------------------------------------------------------------
+24 |  Liter(SemanticArray[SemanticInt.type], x) // error
+   |       ^
+   |       invalid new prefix (dim: Int): SemanticArray[SemanticInt.type] cannot replace ty.type in type ty.T

tests/neg/i16842.scala

@@ -21,5 +21,5 @@ def typecheckArrayLiter(
     a: ArrayLiter
 ): Liter[SemanticArray[SemanticType]] = {
   val x: List[Expr2[SemanticInt.type]] = List()
-  Liter(SemanticArray[SemanticInt.type], x) // error // error
+  Liter(SemanticArray[SemanticInt.type], x) // error
 }

tests/neg/i18058.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i18058.scala:4:21 ----------------------------------------------------------------------------------
+4 |type G = (f: _ <: F) => f.A // error
+  |                     ^
+  |                     invalid new prefix  <: F cannot replace f.type in type f.A

tests/neg/i18058.scala

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