DatatypeContexts in data types and data families

Does datatype contexts only apply for non-phantom variables? Figured I'd ask

{-# Language DatatypeContexts, TypeFamilies #-}

-- F :: Foo a b
data (a ~ b) => Foo a b = F

and

{-# Language DatatypeContexts, KindSignatures, DatatypeContexts, DataKinds, TypeFamilies #-}

data N = O | S N

-- FO :: Fin (S n) n' O
data family   Fin (n::N) (a::N) (b::N)
data instance (n ~ n') => Fin (S n)  n' O =  FO

It surprised me to see the first two of these (yes I know, datatype contexts):

-- F :: x -> Foo x x
data (a ~ b) => Foo a b = F a

-- F :: x -> Foo x x
data (a ~ b) => Foo a b = F b

-- F :: x -> x -> Foo x x
data (a ~ b) => Foo a b = F a b

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture