Fix issue with GADT not typechecking without bind in match

Author: felixmulder

src/dotty/tools/dotc/typer/Typer.scala

@@ -1653,7 +1653,10 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         case SearchSuccess(inferred, _, _) =>
           adapt(inferred, pt)
         case failure: SearchFailure =>
-          if (pt.isInstanceOf[ProtoType] && !failure.isInstanceOf[AmbiguousImplicits]) tree
+          if (
+            pt.isInstanceOf[ProtoType] && !failure.isInstanceOf[AmbiguousImplicits] ||
+            tree.tpe.<:<(pt)(ctx.addMode(Mode.GADTflexible))
+          ) tree
           else err.typeMismatch(tree, pt, failure)
       }
     }

tests/pos/i1307.scala

@@ -0,0 +1,7 @@
+class Term[A]
+class Number(val n: Int) extends Term[Int]
+object Test {
+  def f[B](t: Term[B]): B = t match {
+    case y: Number => y.n
+  }
+}