SI-8079 Only expand local aliases during variance checks

We've been flip-flopping on this one through the years, right
now we issue an two errors for the enclosed test.

After this commit, variance validation only expands aliases that
are `{private,protected}[this]`. The rest need not be expanded,
as we have already variance validated the RHS of the alias.

It also removes a seemingly incorrect check in `isLocalOnly`.

This also means that we can use `@uncheckedVariance` to create
variant type aliases for Java interfaces.

However, if such a type alias is declared private local, it
*will* be expanded. That shouldn't be a problem, other than
for the fact that we run through an as-seen-from that strips
the `@uV` annotations in the type expansion. This has been
recorded in a pending test.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/pending/pos/t8079c.scala b/test/pending/pos/t8079c.scala
new file mode 100644
index 0000000000..ae7f37e2bf
--- /dev/null
+++ b/test/pending/pos/t8079c.scala
@@ -0,0 +1,7 @@
+trait F1[/* - */T, /* + */ R]
+
+object Test {
+  import scala.annotation.unchecked._
+  private[this] type VariantF1[-T, +R] = F1[T @uncheckedVariance, R @uncheckedVariance]
+  trait C[+T] { def foo: VariantF1[Any, T] }
+}