Guard against invalid prefixes in argForParam (#23508)

Fixes #23504

Author: odersky

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

@@ -2304,7 +2304,7 @@ 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
@@ -3069,6 +3069,8 @@ 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:

compiler/test/dotc/neg-best-effort-unpickling.excludelist

@@ -24,3 +24,6 @@ context-function-syntax.scala
 
 # Failure to disambiguate overloaded reference
 i23402b.scala
+
+# Unhandled TypeError exception
+i23504.scala

tests/neg/i23504.scala

@@ -0,0 +1,3 @@
+def test =
+  Seq.empty[[T] =>> () => ?].head() // error
+  Seq.empty[[T] =>> Int => Int].head(1) // error