More systematic handling of invalid prefixes

It looks like there are many situations where an illegal higher-kinded type in an argument for
a value type parameter causes illegal TermRefs and TypeRefs to be constructed, leading to an
assertion error. We now turn the assertion error into a specialized exception which eventually
leads to a TypeError being thrown.

The problem is we cannot detect illegal kinds in arguments early enough to prevent these
situations. We do detect them later, but the damage can already be done before that.

Author: odersky

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

@@ -2302,7 +2302,7 @@ object Types extends TypeUtils {
     def _1: Type
     def _2: Designator
 
-    assert(NamedType.validPrefix(prefix), s"invalid prefix $prefix")
+    if !NamedType.validPrefix(prefix) then throw InvalidPrefix()
 
     private var myName: Name | Null = null
     private var lastDenotation: Denotation | Null = null
@@ -2736,10 +2736,7 @@ object Types extends TypeUtils {
           while (tparams.nonEmpty && args.nonEmpty) {
             if (tparams.head.eq(tparam))
               return args.head match {
-                case _: TypeBounds if !widenAbstract =>
-                  if !NamedType.validPrefix(pre) then
-                    throw TypeError(em"invalid prefix $pre cannot replace parameter $tparam in result of selection")
-                  TypeRef(pre, tparam)
+                case _: TypeBounds if !widenAbstract => TypeRef(pre, tparam)
                 case arg => arg
               }
             tparams = tparams.tail
@@ -3070,6 +3067,8 @@ object Types extends TypeUtils {
       apply(prefix, designatorFor(prefix, name, denot)).withDenot(denot)
   }
 
+  class InvalidPrefix extends Exception
+
   // --- Other SingletonTypes: ThisType/SuperType/ConstantType ---------------------------
 
   /** The type cls.this

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

@@ -3,6 +3,7 @@ package core
 
 import Types.*, Contexts.*, util.Stats.*, Hashable.*, Names.*
 import config.Config
+import Symbols.Symbol
 import Decorators.*
 import util.{WeakHashSet, Stats}
 import WeakHashSet.Entry
@@ -41,8 +42,10 @@ object Uniques:
       val h = doHash(null, designator, prefix)
       if monitored then recordCaching(h, classOf[NamedType])
       def newType =
-        if (isTerm) new CachedTermRef(prefix, designator, h)
-        else new CachedTypeRef(prefix, designator, h)
+        try
+          if isTerm then new CachedTermRef(prefix, designator, h)
+          else new CachedTypeRef(prefix, designator, h)
+        catch case ex: InvalidPrefix => badPrefix(prefix, designator)
       if h == NotCached then newType
       else
         // Inlined from WeakHashSet#put
@@ -61,6 +64,14 @@ object Uniques:
 
         linkedListLoop(oldHead)
       end if
+    end enterIfNew
+
+    private def badPrefix(prefix: Type, desig: Designator)(using Context): Nothing =
+      def name = desig match
+        case desig: Name => desig
+        case desig: Symbol => desig.name
+      throw TypeError(em"invalid prefix $prefix when trying to form $prefix . $name")
+
   end NamedTypeUniques
 
   final class AppliedUniques extends WeakHashSet[AppliedType](Config.initialUniquesCapacity * 2) with Hashable:

tests/neg/i23504.scala

@@ -1,2 +1,3 @@
 def test =
-  Seq.empty[[T] =>> () => ?].head() // error
+  Seq.empty[[T] =>> () => ?].head() // error
+  Seq.empty[[T] =>> Int => Int].head(1) // error