Fix subtyping of types without symbols

Author: smarter

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

@@ -460,7 +460,7 @@ class TypeComparer(initctx: Context) extends DotClass {
           tp1 match {
             case tp1: NamedType =>
               val sym1 = tp1.symbol
-              (if (sym1 eq tp2.symbol) (
+              (if ((sym1 ne NoSymbol) && (sym1 eq tp2.symbol)) (
                    ctx.erasedTypes
                 || sym1.isStaticOwner
                 || { // Implements: A # X  <:  B # X

test/dotc/tests.scala

@@ -87,7 +87,7 @@ class tests extends CompilerTest {
   @Test def neg_t1843 = compileFile(negDir, "t1843", xerrors = 1)
   @Test def neg_t1843_variances = compileFile(negDir, "t1843-variances", xerrors = 1)
   @Test def neg_t2994 = compileFile(negDir, "t2994", xerrors = 2)
-  @Test def neg_subtyping = compileFile(negDir, "subtyping", xerrors = 1)
+  @Test def neg_subtyping = compileFile(negDir, "subtyping", xerrors = 2)
   @Test def neg_variances = compileFile(negDir, "variances", xerrors = 2)
 
   @Test def dotc = compileDir(dotcDir + "tools/dotc", twice)

tests/neg/subtyping.scala

@@ -4,7 +4,11 @@ class B {
 class A extends B
 
 object Test {
-  def test(): Unit = {
+  def test1(): Unit = {
     implicitly[B#X <:< A#X]
   }
+  def test2(): Unit = {
+    val a : { type T; type U } = ???
+    implicitly[a.T <:< a.U]
+  }
 }