core/src/com/google/inject/internal/RealOptionalBinder.java
private void addDirectTypeBinding(Binder binder) {
    binder
        .bind(bindingSelection.getDirectKey())
        .toProvider(new RealDirectTypeProvider<T>(bindingSelection));
  }
private void addDirectTypeBinding(Binder binder) {
    binder
        .bind(bindingSelection.getDirectKey())
        .toProvider(new RealDirectTypeProvider<T>(bindingSelection));
  }
Adds a binding for T. Multiple calls to this are safe, and will be collapsed as duplicate
bindings.