RebindableSyntax causes type errors when 'fail' is not defined, even if not used.

With -XRebindableSyntax and a wildcard pattern on an action, a spurious compiler error occurs if fail is not in scope:

    Not in scope: ‘fail’
    Perhaps you want to add ‘fail’ to the import list in the import of
    ‘Prelude’ (rebind.hs:6:1-53).
   |
27 |     _ <- m1
   |     ^^^^^^^

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE RebindableSyntax  #-}

module Main where

import Prelude (String, print, Maybe (..), error, id)

class MyFunctor f where
    fmap :: (a -> b) -> f a -> f b

class MyApplicative f where
    pure :: a -> f a
    (<*>) :: f (a -> b) -> f a -> f b

class MyMonad m where
    return :: a -> m a
    (>>) :: m a -> m b -> m b
    (>>=) :: m a -> (a -> m b) -> m b
    join :: m (m a) -> m a

-- Uncommenting the following lines allows testCase1 to compile:
-- class MyFail m where
--     fail :: String -> m a

-- But testCase1 will not require a 'MyFail m' constraint.
testCase1 :: MyMonad m => m a -> m ()
testCase1 m1 = do
    _ <- m1
    return ()

testCase2 :: MyMonad m => m a -> m ()
testCase2 m1 = do
    m1
    return ()

In this example, testCase1 will fail to compile until the type class MyFail is uncommented.

As with #13648 (closed), I think this looks like an easy fix before the 8.2.1 release, and I would be happy to submit a patch next week if someone could point me the way.

Trac metadata

Trac field	Value
Version	8.2.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture