Private interfaces in public API files

[josh11b]

This is a question from #931 :
Should we allow a private interface to be declared in an API file to be referenced in a public named constraint available to users?

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[chandlerc]

What are the tradeoffs of allowing this vs. disallowing? Is there any initial suggested direction here or context?

[josh11b]

I think this issue should probably be deferred until this issue becomes more pressing.

[josh11b]

On second thought, I think we should allow this. We already have an example in #1154 of things that can be used as requirements but can't be implemented, and requiring a private interface to be implemented would be a straightforward way to construct such a thing. This would be consistent with what we've already designed and not really a new feature.

[chandlerc]

Leads seem good here too, let's call this decided as "yes we should allow this" as @josh11b's comment indicates.

[jonmeow]

Filed #2003 to track the need for a proposal here.