Fix #4318: Ignore java.lang.Object companion object

java.lang.Object companion object does not exist but has a companion
object. We used to record a dependency on it for incremental compilation
when `java.lang.Object` was imported.

Author: allanrenucci

.drone.yml

@@ -56,9 +56,9 @@ pipeline:
     commands:
       - cp -R . /tmp/5/ && cd /tmp/5/
       - ./project/scripts/sbt sbt-dotty/scripted
-    when:
-      # sbt scripted tests are slow and only run on nightly or deployment
-      event: [ tag, deployment ]
+    # when:
+    #   # sbt scripted tests are slow and only run on nightly or deployment
+    #   event: [ tag, deployment ]
 
   # DOCUMENTATION:
   documentation:

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -313,6 +313,7 @@ private class ExtractDependenciesCollector extends tpd.TreeTraverser { thisTreeT
 
   private def ignoreDependency(sym: Symbol)(implicit ctx: Context) =
     !sym.exists ||
+    sym.unforcedIsAbsent || // ignore java.lang.Object companion object
     sym.is(PackageClass) ||
     sym.isEffectiveRoot ||
     sym.isAnonymousFunction ||

tests/pos/i4318.scala

@@ -0,0 +1 @@
+import java.lang.Object