Merge pull request #3957 from gsps/fix-#3956

Fix #3956 -- Generic signature should box primitive type underlying refinement

Author: allanrenucci

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -281,7 +281,7 @@ object GenericSignatures {
         case _ =>
           val etp = erasure(tp)
           if (etp eq tp) throw new UnknownSig
-          else jsig(etp)
+          else jsig(etp, toplevel, primitiveOK)
       }
     }
     val throwsArgs = sym0.annotations flatMap ThrownException.unapply

tests/pos/i3956.scala

@@ -0,0 +1,4 @@
+object Foo {
+  type RInt = Int { val x: Int }
+  def f(xs: List[Int]): List[RInt] = ???
+}