Fix subtyping to match DOT rule TDECL-<:

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

@@ -736,8 +736,12 @@ class TypeComparer(initctx: Context) extends DotClass {
   def fourthTry(tp1: Type, tp2: Type): Boolean = tp1 match {
     case tp1: TypeRef =>
       tp1.info match {
-        case TypeBounds(lo1, hi1) =>
-          isSubType(hi1, tp2)
+        case TypeBounds(lo1, hi1) => tp2 match {
+          case tp2: NamedType if tp1.name == tp2.name =>
+            isSubType(tp1.info, tp2.info)
+          case _ =>
+            isSubType(hi1, tp2)
+        }
         case _ =>
           (tp1.symbol eq NothingClass) && tp2.isInstanceOf[ValueType] ||
           (tp1.symbol eq NullClass) && tp2.dealias.typeSymbol.isNullableClass

test/dotc/tests.scala

@@ -52,6 +52,7 @@ class tests extends CompilerTest {
   @Test def pos_approximateUnion = compileFile(posDir, "approximateUnion", doErase)
   @Test def pos_tailcall = compileDir(posDir + "tailcall/", doErase)
   @Test def pos_nullarify = compileFile(posDir, "nullarify", "-Ycheck:nullarify" :: doErase)
+  @Test def pos_subtyping = compileFile(posDir, "subtyping", doErase)
 
   @Test def pos_all = compileFiles(posDir, twice)
   @Test def new_all = compileFiles(newDir, twice)

tests/pos/subtyping.scala

@@ -0,0 +1,38 @@
+class C1
+class C2 extends C1
+class C3 extends C2
+class C4 extends C3
+
+object Test {
+  val c11 : { type T >: C1 <: C1 } = ???
+  val c21 : { type T >: C2 <: C1 } = ???
+  val c22 : { type T >: C2 <: C2 } = ???
+  val c31 : { type T >: C3 <: C1 } = ???
+  val c32 : { type T >: C3 <: C2 } = ???
+  val c33 : { type T >: C3 <: C3 } = ???
+  val c41 : { type T >: C4 <: C1 } = ???
+  val c42 : { type T >: C4 <: C2 } = ???
+  val c43 : { type T >: C4 <: C3 } = ???
+  val c44 : { type T >: C4 <: C4 } = ???
+
+  // Test DOT rule TDECL-<:
+  // This is different from scalac
+  def test_tdecl_lt_colon(): Unit = {
+    implicitly[c31.T <:< c41.T]
+    implicitly[c42.T <:< c41.T]
+  }
+
+  // Test DOT rule TSEL-<:
+  def test_tsel_lt_colon(): Unit = {
+    implicitly[c42.T <:< c11.T]
+    implicitly[c42.T <:< c21.T]
+    implicitly[c42.T <:< c22.T]
+  }
+
+  // Test DOT rule <:-TSEL
+  def test_lt_colon_tsel(): Unit = {
+    implicitly[c33.T <:< c32.T]
+    implicitly[c43.T <:< c32.T]
+    implicitly[c44.T <:< c32.T]
+  }
+}