Fix #859

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -199,7 +199,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       }
       compareWild
     case tp2: LazyRef =>
-      isSubType(tp1, tp2.ref)
+      !tp2.evaluating && isSubType(tp1, tp2.ref)
     case tp2: AnnotatedType =>
       isSubType(tp1, tp2.tpe) // todo: refine?
     case tp2: ThisType =>
@@ -299,7 +299,10 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       }
       compareWild
     case tp1: LazyRef =>
-      isSubType(tp1.ref, tp2)
+      // If `tp1` is in train of being evaluated, don't force it
+      // because that would cause an assertionError. Return false instead.
+      // See i859.scala for an example where we hit this case.
+      !tp1.evaluating && isSubType(tp1.ref, tp2)
     case tp1: AnnotatedType =>
       isSubType(tp1.tpe, tp2)
     case AndType(tp11, tp12) =>

src/dotty/tools/dotc/core/Types.scala

@@ -1972,14 +1972,15 @@ object Types {
   case class LazyRef(refFn: () => Type) extends UncachedProxyType with ValueType {
     private var myRef: Type = null
     private var computed = false
-    lazy val ref = {
+    def ref = {
       if (computed) assert(myRef != null)
       else {
         computed = true
         myRef = refFn()
       }
       myRef
     }
+    def evaluating = computed && myRef == null
     override def underlying(implicit ctx: Context) = ref
     override def toString = s"LazyRef($ref)"
     override def equals(other: Any) = other match {

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 = ???
+}