Trex illegal? overlapping instances for derived classes -- is this intended?

[Anthony Clayden]

For derived instances I'm getting decls that aren't allowed if I try to put
them explicitly. They're rejected as overlapping, and in no
substitution ordering.

>    import Hugs.Trex
>
>    type UserId = Int
>    type EventId = Int
>
>    data Message
>      = SendChat (Rec ( message :: String,  user :: UserId ))
>      | Signup   (Rec ( eventId :: EventId, user :: UserId ))
>      deriving (Eq)
>

There's an `instance Eq Message` -- fine.

>From Hugs.Trex I get (and these are fine)

>    instance EqRecRow a => Eq (Rec a)                -- pass through to a
specialised class
>    instance EqRecRow EmptyRow

These are generated by the `deriving`; note they're not specialised to the
field's type, nor for rows with combinations of labels:

>      instance (Eq a, b\eventId, EqRecRow b) => EqRecRow (eventId :: a | b)
>      instance (Eq a, b\user, EqRecRow b) => EqRecRow (user :: a | b)
>      instance (Eq a, b\message, EqRecRow b) => EqRecRow (message :: a | b)

Not specialising and not combining labels makes sense: then no clash if I
also had:

>    data Probe a b c
>      = MkProbe (Rec (user :: a, message :: b, eventId :: c))
>      deriving (Eq)

But those generated instances for `EqRecRow` overlap in no substitution
ordering -- as Hugs tells me if I try to declare them explicitly. How does
a wanted for `(MkProbe x) == (MkProbe y)` resolve to the wanted instances?
There's no preference ordering -- or is there?

Precisely because it's Trex where records are considered isomorphic up to
permutation of labels; and because each instance has an `EqRecRow b`
constraint for the rest of the row; it doesn't matter how Hugs resolves the
wanted to the instances: it'll get to the same answer. But I don't see how
this fits with Hugs usual approach for overlaps.

AntC
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