SI-7126 Account for the alias types that don't dealias.

After this change:

    qbin/scalac -Ydebug test/files/pos/t7126.scala 2>&1 | grep warning
    warning: dropExistential did not progress dealiasing Test.this.T[Test.this.T], see SI-7126
    one warning found

T[T]? Really? The true bug lies somewhere else; the comments of
the ticket illuminate the general areas of concern.

1 files changed, 7 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index cff3f4f0fa..4fb68fc2a9 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -222,8 +222,13 @@ trait Typers extends Modes with Adaptations with Tags {
         new SubstWildcardMap(tparams).apply(tp)
       case TypeRef(_, sym, _) if sym.isAliasType =>
         val tp0 = tp.dealias
-        val tp1 = dropExistential(tp0)
-        if (tp1 eq tp0) tp else tp1
+        if (tp eq tp0) {
+          debugwarn(s"dropExistential did not progress dealiasing $tp, see SI-7126")
+          tp
+        } else {
+          val tp1 = dropExistential(tp0)
+          if (tp1 eq tp0) tp else tp1
+        }
       case _ => tp
     }