Template Haskell can't derive Read, Show, Eq instances

Given the following code:

import Language.Haskell.TH
makeAlpha' n = [d| data Alpha = Alpha $(conT n) deriving (Show, Read) |]

The compiler complains with the following error:

Can't derive instances where the instance context mentions
type variables that are not data type parameters
  Offending constraint: Show t_d

But manually constructing the AST works without issues:

makeAlpha n = do
  let alpha = mkName "Alpha"
  dec <- dataD (return []) alpha []
    [normalC alpha [fmap ((,) NotStrict) $ conT n]]
    [''Show, ''Read]
  return [dec]

Trac metadata

Trac field	Value
Version	7.0.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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