OVERLAPPABLE/OVERLAPPING/OVERLAPS pragmas are confusing

[Johan Tibell]

It was brought to my attention that cassava, my library,
uses OverlappingInstances, which is now deprecated. There's a suggested fix
here: https://github.com/tibbe/cassava/pull/95.

The fix seems correct but, as Mikhail points out, makes some client code no
longer compile (due to a now missing OVERLAPPABLE pragma).

What's the right way to migrate code? Just switching my library to the new
pragmas breaks code, so that doesn't seem very attractive. Do clients have
to migrate before the libraries they use?

-- Johan
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