SI-7984 Issue unchecked warning for type aliases

src/compiler/scala/tools/nsc/typechecker/Checkable.scala

@@ -259,10 +259,12 @@ trait Checkable {
       if (uncheckedOk(P0)) return
       def where = if (inPattern) "pattern " else ""
 
-      // singleton types not considered here
-      val P = P0.widen
+      // singleton types not considered here, dealias the pattern for SI-XXXX
+      val P = P0.dealiasWiden
       val X = X0.widen
 
+      def PString = if (P eq P0) P.toString else s"$P (the underlying of $P0)"
+
       P match {
         // Prohibit top-level type tests for these, but they are ok nested (e.g. case Foldable[Nothing] => ... )
         case TypeRef(_, NothingClass | NullClass | AnyValClass, _) =>
@@ -282,12 +284,12 @@ trait Checkable {
 
           if (checker.neverMatches) {
             val addendum = if (checker.neverSubClass) "" else " (but still might match its erasure)"
-            getContext.unit.warning(tree.pos, s"fruitless type test: a value of type $X cannot also be a $P$addendum")
+            getContext.unit.warning(tree.pos, s"fruitless type test: a value of type $X cannot also be a $PString$addendum")
           }
           else if (checker.isUncheckable) {
             val msg = (
-              if (checker.uncheckableType =:= P) s"abstract type $where$P"
-              else s"${checker.uncheckableMessage} in type $where$P"
+              if (checker.uncheckableType =:= P) s"abstract type $where$PString"
+              else s"${checker.uncheckableMessage} in type $where$PString"
             )
             getContext.unit.warning(tree.pos, s"$msg is unchecked since it is eliminated by erasure")
           }

test/files/neg/t3692-new.check

@@ -1,10 +1,10 @@
-t3692-new.scala:14: warning: non-variable type argument Int in type pattern Map[Int,Int] is unchecked since it is eliminated by erasure
+t3692-new.scala:14: warning: non-variable type argument Int in type pattern scala.collection.immutable.Map[Int,Int] (the underlying of Map[Int,Int]) is unchecked since it is eliminated by erasure
       case m0: Map[Int, Int] => new java.util.HashMap[Integer, Integer]
                ^
-t3692-new.scala:15: warning: non-variable type argument Int in type pattern Map[Int,V] is unchecked since it is eliminated by erasure
+t3692-new.scala:15: warning: non-variable type argument Int in type pattern scala.collection.immutable.Map[Int,V] (the underlying of Map[Int,V]) is unchecked since it is eliminated by erasure
       case m1: Map[Int, V] => new java.util.HashMap[Integer, V]
                ^
-t3692-new.scala:16: warning: non-variable type argument Int in type pattern Map[T,Int] is unchecked since it is eliminated by erasure
+t3692-new.scala:16: warning: non-variable type argument Int in type pattern scala.collection.immutable.Map[T,Int] (the underlying of Map[T,Int]) is unchecked since it is eliminated by erasure
       case m2: Map[T, Int] => new java.util.HashMap[T, Integer]
                ^
 t3692-new.scala:15: warning: unreachable code

test/files/neg/t7984.check

@@ -0,0 +1,6 @@
+t7984.scala:4: warning: non-variable type argument Int in type pattern List[Int] (the underlying of Test.this.ListInt) is unchecked since it is eliminated by erasure
+    case is: ListInt => is.head
+             ^
+error: No warnings can be incurred under -Xfatal-warnings.
+one warning found
+one error found

test/files/neg/t7984.flags

@@ -0,0 +1 @@
+-Xfatal-warnings

test/files/neg/t7984.scala

@@ -0,0 +1,7 @@
+class Test {
+  type ListInt = List[Int]
+  List[Any]("") match {
+    case is: ListInt => is.head
+    case _ =>
+  }
+}

test/files/neg/unchecked-suppress.check

@@ -1,7 +1,7 @@
-unchecked-suppress.scala:4: warning: non-variable type argument Int in type pattern Set[Int] is unchecked since it is eliminated by erasure
+unchecked-suppress.scala:4: warning: non-variable type argument Int in type pattern scala.collection.immutable.Set[Int] (the underlying of Set[Int]) is unchecked since it is eliminated by erasure
     case xs: Set[Int]                              => xs.head   // unchecked
              ^
-unchecked-suppress.scala:5: warning: non-variable type argument String in type pattern Map[String @unchecked,String] is unchecked since it is eliminated by erasure
+unchecked-suppress.scala:5: warning: non-variable type argument String in type pattern scala.collection.immutable.Map[String @unchecked,String] (the underlying of Map[String @unchecked,String]) is unchecked since it is eliminated by erasure
     case xs: Map[String @unchecked, String]        => xs.head   // one unchecked, one okay
              ^
 unchecked-suppress.scala:7: warning: non-variable type argument Int in type pattern (Int, Int) => Int is unchecked since it is eliminated by erasure

test/files/neg/unchecked.check

@@ -1,10 +1,10 @@
-unchecked.scala:18: warning: non-variable type argument String in type pattern Iterable[String] is unchecked since it is eliminated by erasure
+unchecked.scala:18: warning: non-variable type argument String in type pattern Iterable[String] (the underlying of Iterable[String]) is unchecked since it is eliminated by erasure
     case xs: Iterable[String] => xs.head // unchecked
              ^
-unchecked.scala:22: warning: non-variable type argument Any in type pattern Set[Any] is unchecked since it is eliminated by erasure
+unchecked.scala:22: warning: non-variable type argument Any in type pattern scala.collection.immutable.Set[Any] (the underlying of Set[Any]) is unchecked since it is eliminated by erasure
     case xs: Set[Any] => xs.head // unchecked
              ^
-unchecked.scala:26: warning: non-variable type argument Any in type pattern Map[Any,Any] is unchecked since it is eliminated by erasure
+unchecked.scala:26: warning: non-variable type argument Any in type pattern scala.collection.immutable.Map[Any,Any] (the underlying of Map[Any,Any]) is unchecked since it is eliminated by erasure
     case xs: Map[Any, Any] => xs.head // unchecked
              ^
 unchecked.scala:35: warning: non-variable type argument List[Nothing] in type pattern Test.Contra[List[Nothing]] is unchecked since it is eliminated by erasure

test/files/neg/unchecked3.check

@@ -31,7 +31,7 @@ unchecked3.scala:62: warning: non-variable type argument Array[String] in type p
 unchecked3.scala:63: warning: non-variable type argument String in type pattern Array[Array[List[String]]] is unchecked since it is eliminated by erasure
     /*   warn */ case _: Array[Array[List[String]]] => ()
                          ^
-unchecked3.scala:75: warning: abstract type A in type pattern Set[Q.this.A] is unchecked since it is eliminated by erasure
+unchecked3.scala:75: warning: abstract type A in type pattern scala.collection.immutable.Set[Q.this.A] (the underlying of Set[Q.this.A]) is unchecked since it is eliminated by erasure
       /*   warn */ case xs: Set[A]  => xs.head
                             ^
 unchecked3.scala:62: warning: unreachable code