SSO enforcement should redirect unauthenticated users to SSO signin page when visiting a private Pages project

Problem to solve

Unauthenticated user's are redirected to the generic GitLab sign in page when visiting a Private Pages link (project members only), instead of the SSO sign in link when SSO enforcement is on.

Proposal

When SSO enforcement is enabled, automatically redirect unauthenticated users to the SSO sign in link when visiting a private Pages page.

User experience goal

To make it easier for unauthenticated users (or to remind them) to login to GitLab via their SSO signin.

Otherwise, they may

• Sign in twice (once for the generic GitLab sign in page, then a second time with the SSO signin because SSO enforcement is enabled).
• Or have to manually go looking for the SSO signin link.

Further details

Redirecting to the SSO signin works as expected on non-Pages links

What does success look like, and how can we measure that?

Unauthenticated user's are able to easily login via SSO when visiting a private Pages page when SSO enforcement is on.

Links / references

Customer Request request (internal ZD link): https://gitlab.zendesk.com/agent/tickets/202203