Revert "Heal member-select on opaque reference" (#21341)

This reverts commit 4443395.

This reverts the behaviour changing part of #19730, which could make
code that previously compiled, continue to
compile but with a different runtime behaviour. Now, the code in
question is very niche and esoteric, and
effectively exploiting this opaque typing bug. So, we're happy to change
the compiler to make this behaviour
silently change, just not in a patch release.

(Same as #21340, but for the LTS.)

Author: WojciechMazur

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

@@ -1494,7 +1494,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
    *  Note: It would be legal to do the lifting also if M does not contain opaque types,
    *  but in this case the retries in tryLiftedToThis would be redundant.
    */
-  def liftToThis(tp: Type): Type = {
+  private def liftToThis(tp: Type): Type = {
 
     def findEnclosingThis(moduleClass: Symbol, from: Symbol): Type =
       if ((from.owner eq moduleClass) && from.isPackageObject && from.is(Opaque)) from.thisType

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -715,18 +715,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         typedSelectWithAdapt(tree, pt, qual)
       else EmptyTree
 
-    // Otherwise, heal member selection on an opaque reference,
-    // reusing the logic in TypeComparer.
-    def tryLiftToThis() =
-      val wtp = qual.tpe.widen
-      val liftedTp = comparing(_.liftToThis(wtp))
-      if liftedTp ne wtp then
-        val qual1 = qual.cast(liftedTp)
-        val tree1 = cpy.Select(tree0)(qual1, selName)
-        val rawType1 = selectionType(tree1, qual1)
-        tryType(tree1, qual1, rawType1)
-      else EmptyTree
-
     // Otherwise, map combinations of A *: B *: .... EmptyTuple with nesting levels <= 22
     // to the Tuple class of the right arity and select from that one
     def trySmallGenericTuple(qual: Tree, withCast: Boolean) =
@@ -793,7 +781,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     tryType(tree, qual, rawType)
       .orElse(trySimplifyApply())
       .orElse(tryInstantiateTypeVar())
-      .orElse(tryLiftToThis())
       .orElse(trySmallGenericTuple(qual, withCast = true))
       .orElse(tryExt(tree, qual))
       .orElse(tryGadt())