SI-7984 Issue unchecked warning for type aliases

 - Dealias pattern types before launching the CheckabilityChecker
 - Sharpen the error messages to explain that the dealiased type
   is the expansion of the alias.

1 files changed, 7 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala b/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
index 0eae17612d..94f8f509fc 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Checkable.scala
+++ b/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")
           }