Let's rework Data.Type.Equality.==

[Ryan Scott]

Personally, I'd be more inclined towards latter approach (in
https://phabricator.haskell.org/D3837). After all, one of the key
properties of (==) (for which the Haddocks even make a special note)
is that it does not attempt to provide an instance that works for
every possible kind. Indeed, as you've discovered, there are cases
when it doesn't make sense to use DefaultEq, such as for ().

I'll tentatively give my support for D3837, although I'd be curious to
hear what Richard has to say about this (since I'm reasonably
confident he gave (==) its current implementation).

Ryan S.