GHC HEAD accepts a manual instance decl but not equivalent TH decl

If I try to build the traverse-with-class package with GHC HEAD, I get an error:

Data/Generics/Traversable/Instances.hs:21:1:
    Illegal type constructor or class name: ‘c’
    When splicing a TH declaration:
      instance c_0 a_1 => Data.Generics.Traversable.Core.GTraversable c_0
                                                                (Data.Maybe.Maybe a_1)
    where Data.Generics.Traversable.Core.gtraverse = \f_2 x_3 -> case x_3 of
                                                                     Data.Maybe.Nothing -> Control.Applicative.pure Data.Maybe.Nothing
                                                                     Data.Maybe.Just arg_4 -> (Control.Applicative.<*>) (Control.Applicative.pure Data.Maybe.Just) (f_2 arg_4)

This code used to compile with all the previous versions. Additionally, if I replace the TH splice with the generated instance declaration (with minimal corrections to make all names resolve), it is accepted:

instance c_0 a_1 => Data.Generics.Traversable.Core.GTraversable c_0
                                                                (Maybe a_1)
    where gtraverse = \f_2 x_3 -> case x_3 of
                                                                     Nothing -> Control.Applicative.pure Nothing
                                                                     Just arg_4 -> (Control.Applicative.<*>) (Control.Applicative.pure Just) (f_2 arg_4)

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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