Unable to resolve type families

The following code

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies     #-}

import Control.Monad.Classes (MonadReader)
--import Control.Monad.Primitive ()
import Control.Monad.Trans.State.Lazy (StateT)

main :: (n ~ StateT () IO, MonadReader () n) => IO ()
main = undefined

produces the error (cleaned up for readability)

Main.hs:9:1: error:
    * No instance for 
        (monad-classes.MonadReaderN
           (monad-classes.FindTrue
              '[monad-classes.CanDo
                  (StateT () IO)
                  (monad-classes.EffReader ()),
                monad-classes.CanDo
                  IO
                  (monad-classes.EffReader ())])
            ()
            (StateT () IO))
        arising from a use of `main'
    * In the expression: main
      When checking the type of the IO action `main'
  |
9 | main = undefined
  | ^

This is the same error as #16211, but that ticket uses a slightly different example to trigger the error upon recompile.

The relevant instances are all available in monad-classes.

class Monad m => MonadReaderN (n :: Peano) r m
instance Monad m => MonadReaderN Zero r (StateT r m)

type family CanDo (m :: (* -> *)) (eff :: k) :: Bool
type instance CanDo (StateT s m) eff = StateCanDo s eff

type family StateCanDo s eff where
  StateCanDo s (EffState s) = True
  StateCanDo s (EffReader s) = True

type family FindTrue (bs :: [Bool]) :: Peano where
  FindTrue (True ': t) = Zero
  FindTrue (False ': t) = Succ (FindTrue t)

data EffReader (e :: *)

I can reproduce with the attached package using GHC-8.6.3 and cabal:

$ cabal sandbox init
$ cabal install --only-dependencies
$ cabal build

I was unable to minimize the attached example any further. Here's two ways to make the error go away:

1. import Control.Monad.Primitive (). It's unclear why this would help because according to the maintainer of monad-classes, it has no transitive dependencies on primitive.
2. Change the signature of main to main :: (MonadReader () (StateT () IO) => IO ()

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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