Compute the original qualifier type before erasure

The original qualifier type was computed in the wrong context. For the logic
to make sense, it needs to compute the type before erasure and then erase that
type. That fixes #17391 by now compiling it correctly, the same way Scala 2 does.

Author: odersky

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -764,7 +764,9 @@ object Erasure {
         val symIsPrimitive = sym.owner.isPrimitiveValueClass
 
         def originalQual: Type =
-          erasure(tree.qualifier.typeOpt.widen.finalResultType)
+          erasure(
+            inContext(preErasureCtx):
+              tree.qualifier.typeOpt.widen.finalResultType)
 
         if (qualIsPrimitive && !symIsPrimitive || qual.tpe.widenDealias.isErasedValueType)
           recur(box(qual))