hs-boot files don't support boring classes

Summary

A class declaration in an hs-boot file with no constraints and no methods is considered to be abstract, and modules importing the class with a {-# SOURCE #-} import cannot instantiate it. Unfortunately, this heuristic isn't quite right: it's sometimes useful to have an actual class with no constraints or methods, especially with FunctionalDependencies. The obvious work-around is to give the class an explicitly empty constraint in the hs-boot file, but this fails in a somewhat surprising fashion.

Steps to reproduce

-- Fish.hs
{-# language FunctionalDependencies #-}

module Fish where

import Shark

class Guppy a b | a -> b

-- Fish.hs-boot
{-# language FunctionalDependencies #-}

module Fish where

class () => Guppy a b | a -> b

-- Coral.hs
{-# language MultiParamTypeClasses #-}
module Coral where
import {-# SOURCE #-} Fish
import Data.Proxy

data Hoop

instance Guppy Hoop Char

yeah :: Guppy a b => proxy a -> Proxy b
yeah _ = Proxy

-- Shark.hs
module Shark where

import {-# SOURCE #-} Fish
import Coral
import Data.Proxy

goom :: Proxy Hoop -> Proxy Char
goom = yeah

Expected behavior

I expected to get an error about the constraints in the hs-boot file not matching the one in the hs file (since one has no constraints and the other has an empty constraint tuple). I hoped that GHC would understand the hs-boot declaration to mean that the class itself actually has no constraints (and therefore no methods). What I actually got is just silly:

Coral.hs:8:10: error:
    • Cannot define instance for abstract class ‘Guppy’
    • In the instance declaration for ‘Guppy Hoop Char’
  |
8 | instance Guppy Hoop Char

So when GHC sees the Guppy declaration in Fish.hs-boot, it simply erases the () => before even deciding whether the declaration is abstract. This really makes no sense! Either () => is the same as not having constraints (in which case the declaration in the hs-boot file should be considered complete) or it is not the same (in which case I should get a complaint about mismatched declarations). I guess a third possibility is that they currently are the same, but that the GHC devs don't commit to them remaining so. If that's the case, I would also expect a mismatch error, but I would be a bit annoyed.

Environment

• GHC version used:

Optional:

• Operating System:
• System Architecture: