Class restrictions on type instances

I'm using type families to implement bools and integers on type level.

There is a class BoolT with with two instances and type families representing usual boolean functions, e.g. NotT. By semantics of NotT, implication (BoolT a) => (BoolT (NotT a)) holds, but I cannot specify this to compiler, and using BoolT with NotT is painful.

So I think there must be syntactic construction to restrict type family instances to some class, something like

type family NotT a 
    with (BoolT a) => (BoolT (NotT a))

or

class BoolT a where
    type NotT a
        with BoolT (NotT a)

Trac metadata

Trac field	Value
Version	6.10.4
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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