GHC forbids kind signatures in data family instances

If I say

class C (a :: k)
instance (C :: Type -> Constraint) a

GHC is happy. Note that this is actually useful to do.

But if I say

data family D (a :: k)
data instance (D :: Type -> Type) a

GHC is unhappy:

    Malformed head of type or class declaration: (D :: Type -> Type) a

Fixing this will require a change to the FamEqn type, which doesn't have enough flexibility to deal with a kind signature there.

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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changed weight to 5

added Tbug Trac import labels

Should we also permit data (D :: Type -> Type) a?

No. data is a declaration (binding site) and has its own, restrictive syntax. On the other hand, the head of a data instance is a type, containing only occurrences, not binding sites. (Even the type variables are occurrences. They are magically and invisibly bound by an implicit forall.)

added Pnormal label