Guard against NoSymbol in compareGADT

Without the guard the test case crashes with an assert attempting get
the owner of NoSymbol.

Author: milessabin

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

@@ -678,7 +678,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
                 narrowGADTBounds(tp1, tp2, approx, isUpper = true)) &&
                 GADTusage(tp1.symbol)
             }
-            isSubType(hi1, tp2, approx.addLow) || compareGADT
+            isSubType(hi1, tp2, approx.addLow) || ((tp1.symbol ne NoSymbol) && compareGADT)
           case _ =>
             def isNullable(tp: Type): Boolean = tp.widenDealias match {
               case tp: TypeRef => tp.symbol.isNullableClass

tests/pos/dep-match.scala

@@ -0,0 +1,9 @@
+object Test {
+  type Foo = Int { type U }
+  type Bar[T] = T match {
+    case Unit => Unit
+  }
+  inline def baz(foo: Foo): Unit = {
+    val v: Bar[foo.U] = ???
+  }
+}