Allow interfaces to implement other interfaces

At a glance

• Identifier: #373
• Stage: RFC3: Accepted
• Champion: @mike-marcacci
• PR: Allow interfaces to implement other interfaces
• Related:
  • #939 (allow unions to declare implementation of interfaces)

Timeline

• 5 commits pushed on 2020-01-11:
  • factor implementation validity check into reusable algo by @leebyron
  • Expand section on Validation with example of now-valid spread by @leebyron
  • algorithm cleanups by @leebyron
  • Add explaining note about not implementing itself by @leebyron
  • Fix broken reference by @leebyron
• 3 commits pushed on 2020-01-10:
  • Merge branch 'master' into rfc-interfaces-implement-interfaces by @leebyron
  • Apply suggestions from code review by @leebyron
  • Move implementing interfaces into "super-set of all interfaces it imp… by @leebyron
• Added to 2019-11-07 WG agenda
• Mentioned in 2019-11-07 WG notes
• Commit pushed: Use simpler language for cycles on 2019-09-26 by @mike-marcacci
• Commit pushed: fix another typo on 2019-09-16 by @mike-marcacci
• 2 commits pushed on 2019-09-15:
  • Forbid cyclic references by @mike-marcacci
  • Fix small typos by @mike-marcacci
• Added to 2019-09-12 WG agenda
• Mentioned in 2019-09-12 WG notes
• Commit pushed: Add language to require declaring transitive interfaces on 2019-07-23 by @mike-marcacci
• Commit pushed: Merge branch 'master' into rfc-interfaces-implement-interfaces on 2019-07-16 by @mike-marcacci
• Added to 2019-07-03 WG agenda
• Spec PR created on 2017-11-01 by mike-marcacci
• Commit pushed: update introspection and validation on 2017-11-01 by @mike-marcacci

---

This fixes #295 by describing the way in which an interface may implement another interface. The rules are essentially identical to the rules governing implementation of an interface by an object. I'd be more than happy to champion this change (per the CONTRIBUTING.md process) so please direct any questions my way.

This is a pretty small change, but adds substantially to the expressiveness of GraphQL's type system.

If someone in the WG can give me a nod, I'll go ahead and implement it in graphql-js.

Thanks in advance!