Nullary Typeclasses Always Require an Instance?

Summary

In GHCI, at least, creating a nullary typeclass of form "class Foo" does not allow you to use it as a constraint without defining a corresponding instance of Foo.

Actually, we've been discussing this on IRC (esoteric features of the type system!).

You're allowed to define a name with a Foo constraint in the type, but it creates type errors if you attempt to use it, unless you define an instance.

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Also, apparently :t for such a name fails in GHCI. That's what got int-e interested in this.

Steps to reproduce

Load ghci enter "class Foo" enter "u :: Foo => Int; u = 3" enter "u" enter ":t u"

alternately:

Foo.hs

class Foo

u :: Foo => Int u = 3

main = print u

and attempt to compile with GHC

Expected behavior

Allow automatic instancing of nullary typeclasses?

Environment

• GHC version used: 9.4.4

Optional:

• Operating System:
• System Architecture: