Fix scala#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), this is a rare case because `ThisType` are
interned, so usually two `ThisType` to the same class are `eq` and
therefore `isSubType` returns true.

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 =>
+              tp1.cls eq cls2
             case tp1: NamedType if cls2.is(Module) && cls2.eq(tp1.typeSymbol) =>
               cls2.isStaticOwner ||
               recur(tp1.prefix, cls2.owner.thisType) ||

stdlib-bootstrapped-tasty-tests/test/BootstrappedStdLibTASYyTest.scala

@@ -149,55 +149,6 @@ object BootstrappedStdLibTASYyTest:
     "scala.Long",
     "scala.Short",
     "scala.Unit",
-
-    // See #9994
-    // -- Error:
-    //     |  def addOne(kv: (K, V)) = {
-    //     |      ^
-    //     |error overriding method addOne in trait Growable of type (elem: (K, V)): (TrieMap.this : scala.collection.concurrent.TrieMap[K, V]);
-    //     |  method addOne of type (kv: (K, V)): (TrieMap.this : scala.collection.concurrent.TrieMap[K, V]) has incompatible type
-    // -- Error:
-    //     |  def subtractOne(k: K) = {
-    //     |      ^
-    //     |error overriding method subtractOne in trait Shrinkable of type (elem: K): (TrieMap.this : scala.collection.concurrent.TrieMap[K, V]);
-    //     |  method subtractOne of type (k: K): (TrieMap.this : scala.collection.concurrent.TrieMap[K, V]) has incompatible type
-    "scala.collection.concurrent.TrieMap",
-    "scala.collection.immutable.HashMapBuilder",
-    "scala.collection.immutable.HashSetBuilder",
-    "scala.collection.immutable.LazyList",
-    "scala.collection.immutable.ListMapBuilder",
-    "scala.collection.immutable.MapBuilderImpl",
-    "scala.collection.immutable.SetBuilderImpl",
-    "scala.collection.immutable.TreeSeqMap",
-    "scala.collection.immutable.VectorBuilder",
-    "scala.collection.immutable.VectorMapBuilder",
-    "scala.collection.mutable.AnyRefMap",
-    "scala.collection.mutable.ArrayBuilder",
-    "scala.collection.mutable.CollisionProofHashMap",
-    "scala.collection.mutable.LongMap",
-    "scala.collection.mutable.SortedMap",
-    "scala.collection.mutable.StringBuilder",
-    "scala.jdk.AnyAccumulator",
-    "scala.jdk.DoubleAccumulator",
-    "scala.jdk.IntAccumulator",
-    "scala.jdk.LongAccumulator",
-
-    // See #9994
-    // -- Error:
-    //   |  override def filterInPlace(p: A => Boolean): this.type = {
-    //   |               ^
-    //   |error overriding method filterInPlace in trait SetOps of type (p: A => Boolean): (HashSet.this : scala.collection.mutable.HashSet[A]);
-    //   |  method filterInPlace of type (p: A => Boolean): (HashSet.this : scala.collection.mutable.HashSet[A]) has incompatible type
-    "scala.collection.mutable.HashSet",
-
-    // See #9994
-    // -- Error:
-    //   |    def force: this.type = {
-    //   |        ^
-    //   |error overriding method force in class Stream of type => (Cons.this : scala.collection.immutable.Stream.Cons[A]);
-    //   |  method force of type => (Cons.this : scala.collection.immutable.Stream.Cons[A]) has incompatible type
-    "scala.collection.immutable.Stream",
-
   )
 
   /** Set of classes that cannot be loaded from TASTy */

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