Merge pull request #3537 from Blaisorblade/issue/6566

SI-6566, unsoundness with alias variance: document fix

1 files changed, 4 insertions, 3 deletions

diff --git a/src/reflect/scala/reflect/internal/Variances.scala b/src/reflect/scala/reflect/internal/Variances.scala
index a7cac5254c..659bfb5d01 100644
--- a/src/reflect/scala/reflect/internal/Variances.scala
+++ b/src/reflect/scala/reflect/internal/Variances.scala
@@ -65,10 +65,11 @@ trait Variances {
        *  The search proceeds from `base` to the owner of `tvar`.
        *  Initially the state is covariant, but it might change along the search.
        *
-       *  An alias which does not override anything is treated as Bivariant;
+       *  A local alias type is treated as Bivariant;
        *  this is OK because we always expand aliases for variance checking.
-       *  However if it does override a type in a base class, we must assume Invariant
-       *  because there may be references to the type parameter that are not checked.
+       *  However, for an alias which might be externally visible, we must assume Invariant,
+       *  because there may be references to the type parameter that are not checked,
+       *  leading to unsoundness (see SI-6566).
        */
       def relativeVariance(tvar: Symbol): Variance = {
         def nextVariance(sym: Symbol, v: Variance): Variance = (