SI-8324 Fix regression in override checks for sealed classes

adeffda25 changed `Symbol#isEffectivelyFinal` to help the optimizer
by inferring finality within sealed class hierarchies.

However, this change wasn't neccesarily welcome for other clients of
that method. In the enclosed test case, we see that overriding checks
in `RefChecks` regressed.

This commit moves the enhanced version into a new method and
selectively uses it in the optimizer (and the tail call optimizer).

1 files changed, 16 insertions, 0 deletions

diff --git a/test/files/pos/t8234.scala b/test/files/pos/t8234.scala
new file mode 100644
index 0000000000..2cb1562326
--- /dev/null
+++ b/test/files/pos/t8234.scala
@@ -0,0 +1,16 @@
+package p1
+
+private abstract class ProjectDef(val autoPlugins: Any) extends ProjectDefinition
+sealed trait ResolvedProject extends ProjectDefinition {
+  def autoPlugins: Any
+}
+
+sealed trait ProjectDefinition {
+  private[p1] def autoPlugins: Any
+}
+
+
+object Test {
+  // was "error: value autoPlugins in class ProjectDef of type Any cannot override final member"
+  new ProjectDef(null) with ResolvedProject
+}