Surive non-existing sourceModule in Scala2 pickled info.

It seems when unpickling nsc that some module classes come
without a source module. Survive this situation rather than
crashing. i859.scala is an example.

i859 compiles with the patch, but causes a deep subtype when unpickling.
Not sure whether scalac does the same.

Author: odersky

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

@@ -101,10 +101,12 @@ object Scala2Unpickler {
       case cinfo => (Nil, cinfo)
     }
     val ost =
-      if ((selfInfo eq NoType) && (denot is ModuleClass))
+      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
       else selfInfo
-
     denot.info = ClassInfo(denot.owner.thisType, denot.classSymbol, Nil, decls, ost) // first rough info to avoid CyclicReferences
     var parentRefs = ctx.normalizeToClassRefs(parents, cls, decls)
     if (parentRefs.isEmpty) parentRefs = defn.ObjectType :: Nil

test/dotc/tests.scala

@@ -103,6 +103,7 @@ class tests extends CompilerTest {
 
   @Test def pos_i871 = compileFile(posSpecialDir, "i871", scala2mode)
   @Test def pos_variancesConstr = compileFile(posSpecialDir, "variances-constr", scala2mode)
+  @Test def pos_859 = compileFile(posSpecialDir, "i859", scala2mode)(allowDeepSubtypes)
 
   @Test def new_all = compileFiles(newDir, twice)
 

tests/pos-special/i859.scala

@@ -0,0 +1,3 @@
+class Analyzer {
+  def foo: scala.tools.nsc.Global = ???
+}