type t('a) =
  | E: t([ | `Empty])
  | F(string): t([ | `Ready]);

type action =
  | No;

let component = ReasonReact.reducerComponent("Page");

let make = _children => {
  ...component,
  initialState: () => E,
  reducer: (action, state: t('a)) =>
    switch (action) {
    | No => ReasonReact.Update(F("action"))
    },
  render: self => {
    let message: type a. t(a) => string =
      state =>
        switch (state) {
        | E => "empty"
        | F(msg) => msg
        };
    <div> (ReasonReact.stringToElement(message(self.state))) </div>;
  },
};





This has type:
    ReasonReact.componentSpec
    (ReactTemplate.Page.t([ `Ready ]),  ReactTemplate.Page.t([ `Empty ]),
      ReasonReact.noRetainedProps,  ReasonReact.noRetainedProps,
      ReactTemplate.Page.action)
  But somewhere wanted:
    ReasonReact.component (ReactTemplate.Page.t([ `Ready ]),  'a,  'b)
      (defined as
      ReasonReact.componentSpec
      (ReactTemplate.Page.t([ `Ready ]),  ReactTemplate.Page.t([ `Ready ]),
        'a,  'a,  'b))

  The incompatible parts:
    ReactTemplate.Page.t([ `Empty ]) (defined as
      ReactTemplate.Page.t([ `Empty ]))
    vs
    ReactTemplate.Page.t([ `Ready ]) (defined as
      ReactTemplate.Page.t([ `Ready ]))
  These two variant types have no intersection