anchor.diff:  
   /// Indicates if an F# type is the type associated with an F# exception declaration
   let isExnDeclTy g ty =
  -    isAppTy g ty && (tcrefOfAppTy g ty).IsExceptionDecl
  +    match tryTcrefOfAppTy g ty with
  +    | ValueSome tcref -> tcref.IsExceptionDecl
  +    | _ -> false
   
   /// Get the base type of a type, taking into account type instantiations. Return None if the
   /// type has no base type.
   let GetSuperTypeOfType g amap m ty =
   #if !NO_EXTENSIONTYPING
  -    let ty = (if isAppTy g ty && (tcrefOfAppTy g ty).IsProvided then stripTyEqns g ty else stripTyEqnsAndMeasureEqns g ty)
  +    let ty =
  +        match tryTcrefOfAppTy g ty with
  +        | ValueSome tcref when tcref.IsProvided -> stripTyEqns g ty 
  +        | _ -> stripTyEqnsAndMeasureEqns g ty
   #else
       let ty = stripTyEqnsAndMeasureEqns g ty
   #endif
  
positive.code:         if seenThisTycon && allowMultiIntfInst = AllowMultiIntfInstantiations.No then state else
  
          let state =
              if isAppTy g ty then
                  let tcref = tcrefOfAppTy g ty
                  let visitedTycon = Set.add tcref.Stamp visitedTycon
                  visitedTycon, visited.Add (tcref, ty), acc
              else
                  state
  
          if ndeep > 100 then (errorR(Error((FSComp.SR.recursiveClassHierarchy (showType ty)), m)); (visitedTycon, visited, acc)) else
  
negative.code: let private FoldHierarchyOfTypeAux followInterfaces allowMultiIntfInst skipUnref visitor g amap m ty acc =
      let rec loop ndeep ty ((visitedTycon, visited: TyconRefMultiMap<_>, acc) as state) =
  
          let seenThisTycon = 
              match tryTcrefOfAppTy g ty with
              | ValueSome tcref -> Set.contains tcref.Stamp visitedTycon
              | _ -> false
  
          // Do not visit the same type twice. Could only be doing this if we've seen this tycon
          if seenThisTycon && List.exists (typeEquiv g ty) (visited.Find (tcrefOfAppTy g ty)) then state else
  