Enable GADT matching for pattern values

So far, only typed patterns an dunapply had GADT matching.
i1737.scala shows that we need to do the same thing for objects.

Author: odersky

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

@@ -1830,7 +1830,9 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         ctx.typeComparer.GADTused = false
         if (ctx.mode is Mode.Pattern) {
           tree match {
-            case _: RefTree | _: Literal if !isVarPattern(tree) =>
+            case _: RefTree | _: Literal
+            if !isVarPattern(tree) &&
+               !(tree.tpe <:< pt)(ctx.addMode(Mode.GADTflexible)) =>
               checkCanEqual(pt, wtp, tree.pos)(ctx.retractMode(Mode.Pattern))
             case _ =>
           }

tests/pos/i1737.scala

@@ -0,0 +1,11 @@
+object Test {
+  sealed trait Foo[A]
+  case object FooI extends Foo[Int]
+  case class FooS(b: Boolean) extends Foo[String]
+
+  def algFoo[A](foo: Foo[A]): A =
+    foo match {
+      case FooI => 42
+      case FooS(b) => "foo"
+    }
+}