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.

5 files changed, 23 insertions, 1 deletions

diff --git a/test/files/neg/t8079a.check b/test/files/neg/t8079a.check
new file mode 100644
index 0000000000..6bbe78afa6
--- /dev/null
+++ b/test/files/neg/t8079a.check
@@ -0,0 +1,4 @@
+t8079a.scala:3: error: contravariant type I occurs in covariant position in type C.this.X of value b
+  def f2(b: X): Unit
+         ^
+one error found
diff --git a/test/files/neg/t8079a.scala b/test/files/neg/t8079a.scala
new file mode 100644
index 0000000000..4997ea282e
--- /dev/null
+++ b/test/files/neg/t8079a.scala
@@ -0,0 +1,4 @@
+trait C[-I] {
+  private[this] type X = C[I]
+  def f2(b: X): Unit
+}
diff --git a/test/files/neg/variances.check b/test/files/neg/variances.check
index cb1a60a632..3c1545a375 100644
--- a/test/files/neg/variances.check
+++ b/test/files/neg/variances.check
@@ -19,7 +19,7 @@ variances.scala:74: error: covariant type A occurs in contravariant position in
 variances.scala:89: error: covariant type T occurs in invariant position in type T of type A
     type A = T
          ^
-variances.scala:90: error: covariant type T occurs in contravariant position in type => test.TestAlias.B[C.this.A] of method foo
+variances.scala:90: error: covariant type A occurs in contravariant position in type => test.TestAlias.B[C.this.A] of method foo
     def foo: B[A]
         ^
 8 errors found
diff --git a/test/files/pos/t8079b.scala b/test/files/pos/t8079b.scala
new file mode 100644
index 0000000000..f3b7b78077
--- /dev/null
+++ b/test/files/pos/t8079b.scala
@@ -0,0 +1,7 @@
+trait F1[/* - */T, /* + */ R]
+
+object Test {
+  import scala.annotation.unchecked._
+  type VariantF1[-T, +R] = F1[T @uncheckedVariance, R @uncheckedVariance]
+  trait C[+T] { def foo: VariantF1[Any, T] }
+}
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] }
+}