class declaration works in ghci, but not in a file

The following ghci session prints no errors:

> :set -XStandaloneDeriving -XDeriveGeneric
> import GHC.Generics
> import Control.Monad.Except
> deriving instance Generic (ExceptT e m a)
> class F a where f :: Rep (Except String a) x

However, when I transfer this to a file:

{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
import Control.Monad.Except
deriving instance Generic (ExceptT e m a)
class F a where f :: Rep (Except String a) x

I get a mysterious error:

test.hs:6:17: error:
    • Couldn't match type ‘Rep (Except String a0)’
                     with ‘Rep (Except String a)’
      Expected type: Rep (Except String a) x
        Actual type: Rep (Except String a0) x
      NB: ‘Rep’ is a type function, and may not be injective
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘f’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        f :: forall a. F a => forall x. Rep (Except String a) x
      In the class declaration for ‘F’

If I turn on -fdefer-type-errors, I can verify that the type family indeed reduces far enough for a and x to be arguments to injective types, so I believe GHC should not consider this an error:

> :set -XRankNTypes
> :kind! forall a x. Rep (Except String a) x
forall a x. Rep (Except String a) x :: *
= D1
    ('MetaData
       "ExceptT"
       "Control.Monad.Trans.Except"
       "transformers-0.5.2.0"
       'True)
    (C1
       ('MetaCons "ExceptT" 'PrefixI 'False)
       (S1
          ('MetaSel
             'Nothing 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy)
          (Rec0 (Data.Functor.Identity.Identity (Either [Char] a)))))
    x

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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