# Possible regression in Coercible a (X a) between 7.8 and 7.10

In upgrading to7.10, code of the form

```
{-# LANGUAGE FlexibleContexts #-}
import Data.Coerce (coerce, Coercible)
data X a
doCoerce :: Coercible a (X a) => a -> X a
doCoerce = coerce
```

fails to compile in 7.10.1 and 7.10.2 with the error

```
testCoerce.hs:6:13:
Could not deduce (a ~ X a)
from the context (Coercible a (X a))
bound by the type signature for
doCoerce :: Coercible a (X a) => a -> X a
at testCoerce.hs:6:13-41
‘a’ is a rigid type variable bound by
the type signature for doCoerce :: Coercible a (X a) => a -> X a
at testCoerce.hs:6:13
Relevant role signatures: type role X phantom
In the ambiguity check for the type signature for ‘doCoerce’:
doCoerce :: forall a. Coercible a (X a) => a -> X a
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature for ‘doCoerce’:
doCoerce :: Coercible a (X a) => a -> X a
```

while it works in 7.8.4.

Surprisingly (to me at least), the code works in 7.10.1 and 7.10.2 if I change it to

```
{-# LANGUAGE FlexibleContexts #-}
import Data.Coerce (coerce, Coercible)
data X a
doCoerce :: Coercible a (X b) => a -> X a
doCoerce = coerce
```

while it fails to compile in 7.8.4 with the error

```
testCoerce.hs:6:13:
Could not coerce from ‘a’ to ‘X b0’
because ‘a’ and ‘X b0’ are different types.
arising from the ambiguity check for ‘doCoerce’
from the context (Coercible a (X b))
bound by the type signature for
doCoerce :: Coercible a (X b) => a -> X a
at testCoerce.hs:6:13-41
The type variable ‘b0’ is ambiguous
In the ambiguity check for:
forall a b. Coercible a (X b) => a -> X a
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature for ‘doCoerce’:
doCoerce :: Coercible a (X b) => a -> X a
```

The coercion pattern may look a bit funny, but it is rather useful when one has newtypes of the form

`newtype Y = Y (ForeignPtr Y)`

which appear naturally when writing bindings to C libraries, and one wants to get access to the underlying ForeignPtr from Y (here X is ForeignPtr). The relevant Coercible instance here is Coercible Y (ForeignPtr Y), as above.

I would have expected the version with context "Coercible a (X a)" to be accepted, as 7.8.4 does, since it seems to be a specialization of the more general coerce, but maybe I am missing something?