Should digits be included in smcp?


I am thinking about general typographic rules regarding small caps digits. When I generate smcp and c2sc functions, substitutions for numbers are added automatically in both features, but I am not sure if this is right, because in practice lining digits should be treated as uppercase, so I don’t think that smcp should be affecting them.

Here are some examples how different fonts work with smcp turned on and c2sc off:

So we can see that the issue is treated differently by various designers which surprises me, because one may think that this is not a matter of taste, but a general rule one way or another.

Another thing worth pointing out is that Glyphs’s approach and InDesign’s approach are opposite, because when the font doesn’t include small caps, InDesign doesn’t scale down digits, like it does with uppercase letters to create small caps from lowercase.

Another funny thing is that Minion Pro substitutes digits in smcp, but Minion Variable Concept doesn’t.

What do you think? Which is correct? Should it be a general rule?

There’s some related discussion at this old TypeDrawers thread.