Fix #9916: Properly unpickle non-toplevel Scala 2 objects

Test cases from #9976.

Author: smarter

compiler/src/dotty/tools/dotc/core/NamerOps.scala

@@ -51,10 +51,10 @@ object NamerOps:
       completer.withSourceModule(findModuleBuddy(name.sourceModuleName, scope))
 
   /** Find moduleClass/sourceModule in effective scope */
-  private def findModuleBuddy(name: Name, scope: Scope)(using Context) = {
+  def findModuleBuddy(name: Name, scope: Scope)(using Context) = {
     val it = scope.lookupAll(name).filter(_.is(Module))
     if (it.hasNext) it.next()
     else NoSymbol.assertingErrorsReported(s"no companion $name in $scope")
   }
 
-end NamerOps
+end NamerOps

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -74,11 +74,14 @@ object Scala2Unpickler {
       case cinfo => (Nil, cinfo)
     }
     val ost =
-      if ((selfInfo eq NoType) && denot.is(ModuleClass) && denot.sourceModule.exists)
-        // it seems sometimes the source module does not exist for a module class.
-        // An example is `scala.reflect.internal.Trees.Template$. Without the
-        // `denot.sourceModule.exists` provision i859.scala crashes in the backend.
-        denot.owner.thisType select denot.sourceModule
+      if (selfInfo eq NoType) && denot.is(ModuleClass) then
+        val sourceModule =
+          denot.sourceModule.orElse {
+            // For non-toplevel modules,  `sourceModule` won't be set when completing
+            // the module class, we need to go find it ourselves.
+            NamerOps.findModuleBuddy(cls.name.sourceModuleName, denot.owner.info.decls)
+          }
+        denot.owner.thisType.select(sourceModule)
       else selfInfo
     val tempInfo = new TempClassInfo(denot.owner.thisType, cls, decls, ost)
     denot.info = tempInfo // first rough info to avoid CyclicReferences

sbt-dotty/sbt-test/scala2-compat/i9916a/build.sbt

@@ -0,0 +1,15 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = "2.13.3"
+
+lazy val `i9916a-lib` = (project in file ("lib"))
+  .settings(scalaVersion := scala2Version)
+
+lazy val `i9916a-test` = (project in file ("main"))
+  .dependsOn(`i9916a-lib`)
+  .settings(
+    scalaVersion := scala3Version,
+    // https://github.com/sbt/sbt/issues/5369
+    projectDependencies := {
+      projectDependencies.value.map(_.withDottyCompat(scalaVersion.value))
+    }
+  )

sbt-dotty/sbt-test/scala2-compat/i9916a/lib/i9916a-lib.scala

@@ -0,0 +1,7 @@
+package i9916a
+
+object Lib {
+  trait P
+  object P
+  def P(x: Int): P = ???
+}

sbt-dotty/sbt-test/scala2-compat/i9916a/main/i9916a-test.scala

@@ -0,0 +1,5 @@
+import i9916a.Lib
+
+trait Test {
+  def foo: Lib.P
+}

sbt-dotty/sbt-test/scala2-compat/i9916a/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/scala2-compat/i9916a/test

@@ -0,0 +1 @@
+> i9916a-test/compile

sbt-dotty/sbt-test/scala2-compat/i9916b/build.sbt

@@ -0,0 +1,15 @@
+val scala3Version = sys.props("plugin.scalaVersion")
+val scala2Version = "2.13.3"
+
+lazy val `i9916b-lib` = (project in file ("lib"))
+  .settings(scalaVersion := scala2Version)
+
+lazy val `i9916b-test` = (project in file ("main"))
+  .dependsOn(`i9916b-lib`)
+  .settings(
+    scalaVersion := scala3Version,
+    // https://github.com/sbt/sbt/issues/5369
+    projectDependencies := {
+      projectDependencies.value.map(_.withDottyCompat(scalaVersion.value))
+    }
+  )

sbt-dotty/sbt-test/scala2-compat/i9916b/lib/i9916b-lib.scala

@@ -0,0 +1,7 @@
+package i9916b
+
+trait T {
+  class A
+  object A
+  def A(x: Int): Unit = ???
+}

sbt-dotty/sbt-test/scala2-compat/i9916b/main/i9916b-test.scala

@@ -0,0 +1,5 @@
+import i9916b.T
+
+trait Test {
+  def foo: T
+}

sbt-dotty/sbt-test/scala2-compat/i9916b/project/plugins.sbt

@@ -0,0 +1 @@
+addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % sys.props("plugin.version"))

sbt-dotty/sbt-test/scala2-compat/i9916b/test

@@ -0,0 +1 @@
+> i9916b-test/compile