When an interface has unexported methods, adding a method is minor, not major

[bobg]

When you add a new method to a type, that's a minor-version change: safe for existing callers to use, but providing new behavior for new code that wants to use it.

But when you add a new method M to an interface, it's a major-version change. Why? Because a caller may have defined its own type T implementing the interface, and if they upgrade, T no longer does unless and until it implements M.

However! Some interfaces are not meant to be implemented outside of the modules in which they're defined. Case in point: EnumDescriptor. It prevents callers from implementing that interface by including an unexported method.

Adding a method to such an interface cannot break calling code, and so is not a major-version change.

This PR demotes a method-added-to-an-interface change from Major to Minor in the specific case where the interface in question contains unexported methods and/or is defined in terms of "internal" types that callers cannot access.

Fixes #28.

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)