Type family signatures in indefinite modules

Backpack does not allow type family signatures in indefinite modules. As an example, although GHC does accept:

-- indefinite module
data Foo :: Bool
-- implementing module
type Foo = 'True

It does not accept:

-- indefinite module
type family Foo (b :: Bool) :: Bool
-- implementing module
type family Foo (b :: Bool) :: Bool where
  Foo 'True = 'False
  Foo 'False = 'True

I'm not sure if there is a fundamental reason for this (e.g. a requirement that everything is matchable). It would be helpful for the thing I'm experimenting with if this restriction could be lifted.

Trac metadata

Trac field	Value
Version	8.6.4
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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CC	ezyang
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