Fix a compiler assertion crash

Author: dwijnand

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

@@ -153,7 +153,10 @@ object Substituters:
         if (tp.binder == from) to(tp.paramNum) else tp
       case tp: NamedType =>
         if (tp.prefix `eq` NoPrefix) tp
-        else tp.derivedSelect(substParams(tp.prefix, from, to, theMap))
+        else
+          val pre1 = substParams(tp.prefix, from, to, theMap)
+          if pre1.isValueTypeOrWildcard || (pre1 eq NoPrefix) then tp.derivedSelect(pre1)
+          else tp
       case _: ThisType =>
         tp
       case tp: AppliedType =>

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..."
+}