Fix a compiler assertion crash

Author: dwijnand

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

@@ -2633,7 +2633,8 @@ object Types {
             }
         }
         if (prefix.isInstanceOf[WildcardType]) WildcardType
-        else withPrefix(prefix)
+        else if prefix.isValueTypeOrWildcard || (prefix eq NoPrefix) then withPrefix(prefix)
+        else typer.ErrorReporting.errorType(i"Cannot ($this).deriveSelect($prefix)", ctx.tree)
       }
 
     /** A reference like this one, but with the given symbol, if it exists */

tests/neg/i12448.scala

@@ -0,0 +1,4 @@
+object Main {
+  def mkArray[T <: A]: T#AType // error // error
+  mkArray[Array] // was: "assertion failed: invalid prefix HKTypeLambda..."
+}