[Haskell-cafe] Pattern synonyms and explicit forall.

[coot at coot.me]

Consider the following existential type:

```

data Some where
  Some :: a -> Some
```

The following (bidirectional) pattern synonym is accepted:

```
pattern Any :: a -> Some
pattern Any a = Some a
```

However using an explicit `forall` results in a type error:
```
pattern Any :: forall a. a -> Some
pattern Any a = Some
```

```
Patterns.hs:63:23: error:
    • Couldn't match expected type ‘a’ with actual type ‘a1’
      ‘a1’ is a rigid type variable bound by
        a pattern with constructor: Some :: forall a. a -> Some,
        in a pattern synonym declaration
        at Patterns.hs:63:18-23
      ‘a’ is a rigid type variable bound by
        the signature for pattern synonym ‘Any’
        at Patterns.hs:62:23
    • In the declaration for pattern synonym ‘Any’
    • Relevant bindings include a :: a1 (bound at Patterns.hs:63:23)
   |
63 | pattern Any a <- Some a
   |                       ^
```

This is indpendent whether `ExplicitForAll` or `ScopedTypedVariables` extensions are enabled or not.

Is there a way around it, or is this a missing feature?

Best regards,
Marcin Szamotulski

Sent with ProtonMail Secure Email.
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