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After commit a599abba, GHC HEAD is no longer able to compile the futhark-0.21.8 library on Hackage. Here is a minimized example of the problem:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

type family F x

f :: (F a ~ Maybe a, Show (F a)) => a -> IO ()
f _ = pure ()

g :: (F a ~ Maybe a, Show (F a)) => a -> IO ()
g = go
  where
    go = f

While GHC 9.2 and earlier are able to compile this, HEAD rejects it with:

$ ~/Software/ghc-9.3.20220328/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o ) [Source file changed]

Bug.hs:13:10: error:
    • Could not deduce (Show a) arising from a use of ‘f’
      from the context: (F a ~ Maybe a, Show (F a))
        bound by the type signature for:
                   g :: forall a. (F a ~ Maybe a, Show (F a)) => a -> IO ()
        at Bug.hs:10:1-46
      Possible fix:
        add (Show a) to the context of
          the type signature for:
            g :: forall a. (F a ~ Maybe a, Show (F a)) => a -> IO ()
    • In the expression: f
      In an equation for ‘go’: go = f
      In an equation for ‘g’:
          g = go
            where
                go = f
   |
13 |     go = f
   |          ^

The release notes entries in a599abba didn't appear to mention this kind of program behavior changing, so I thought it best to file a GHC issue about this. @rae, should this be expected to work?