Fix #9994: Handle subtyping between two ThisType

Previously, i9994.scala failed when compiled with `-Ythrough-tasty`
with:

7 |  def foo: this.type = this
  |      ^
  | error overriding method foo in trait Foo of type => (Bar.this : pkg.Bar);
  |   method foo of type => (Bar.this : pkg.Bar) has incompatible type

The two types were pretty-printed the same but were actually:

ExprType(ThisType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),class Bar)))

and

ExprType(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),class Bar)))

Fixed by explicitly handling subtyping between two
`ThisType` (previously we felt through to `fourthTry` were the type of
the lhs was widened).

Author: smarter

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

@@ -310,6 +310,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         def compareThis = {
           val cls2 = tp2.cls
           tp1 match {
+            case tp1: ThisType =>
+              recur(tp1.tref, tp2.tref)
             case tp1: NamedType if cls2.is(Module) && cls2.eq(tp1.typeSymbol) =>
               cls2.isStaticOwner ||
               recur(tp1.prefix, cls2.owner.thisType) ||

tests/pos/i9994.scala

@@ -0,0 +1,7 @@
+package pkg
+
+trait Foo:
+  def foo: this.type
+
+final class Bar extends Foo:
+  def foo: this.type = this