Revert "fix: do not transform Ident to This in PostTyper anymore" (#24059)

Reverts #23899

Related to #23875, #23999

Author: hamzaremmal

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

@@ -482,7 +482,13 @@ object PatternMatcher {
               onSuccess
             )
           }
-        case WildcardPattern() =>
+        // When match against a `this.type` (say case a: this.type => ???),
+        // the typer will transform the pattern to a `Bind(..., Typed(Ident(a), ThisType(...)))`, 
+        // then post typer will change all the `Ident` with a `ThisType` to a `This`.
+        // Therefore, after pattern matching, we will have the following tree `Bind(..., Typed(This(...), ThisType(...)))`.
+        // We handle now here the case were the pattern was transformed to a `This`, relying on the fact that the logic for
+        // `Typed` above will create the correct type test.
+        case WildcardPattern() | This(_) =>
           onSuccess
         case SeqLiteral(pats, _) =>
           matchElemsPlan(scrutinee, pats, exact = true, onSuccess)

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

@@ -391,7 +391,11 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
             checkNotPackage(tree)
           else
             registerNeedsInlining(tree)
-            checkUsableAsValue(tree)
+            val tree1 = checkUsableAsValue(tree)
+            tree1.tpe match {
+              case tpe: ThisType => This(tpe.cls).withSpan(tree.span)
+              case _ => tree1
+            }
         case tree @ Select(qual, name) =>
           registerNeedsInlining(tree)
           if name.isTypeName then