One thing to note in this implementation is I am intentionally extending OAuth2Provider interface with new functions because I am assuming Fosite is usually the one in use. However, I am happy to add a new OAuth2ProviderWithPAR (or some such interface). Fosite would still implement those functions and so, anyone using the native Compose function wouldn't be affected.

The only gotcha is anyone implementing the consumer (like Hydra) would need to add a -

if parProvider, ok := p.(OAuth2ProviderWithPAR); ok {
   parProvider.NewPushedAuthorizationRequest(...)
}

It's just somewhat painful but is more "correct" from a backward compatibility perspective.

Let me know if this is desired and I will make the change.