Handle subtyping of LazyVals that are in train of being evaluated.

Instead of forcing again, and causing an assertion error, back out
assuming that the result is false. Fixes first problem with scala#859.

Author: odersky

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) =>