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Load this file into GHCi:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
module Bug where

data Foo a where
  FooInt :: Foo Int

pattern Bar :: () => (a ~ Int) => Foo a
pattern Bar = FooInt

And attempt to redefine Bar as follows:

$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, 1 module loaded.
λ> pattern Bar = Nothing

<interactive>:1:15: error:
    • Couldn't match expected type ‘p’ with actual type ‘Maybe a0’
        ‘p’ is untouchable
          inside the constraints: a ~ Int
          bound by a pattern with pattern synonym:
                     Bar :: forall a. () => a ~ Int => Foo a,
                   in an equation for ‘pattern’
          at <interactive>:1:9-11
      ‘p’ is a rigid type variable bound by
        the inferred type of pattern :: Foo a -> p at <interactive>:1:1-21
      Possible fix: add a type signature for ‘pattern’
    • In the expression: Nothing
      In an equation for ‘pattern’: pattern Bar = Nothing
    • Relevant bindings include
        pattern :: Foo a -> p (bound at <interactive>:1:1)

There are two issues here:

1. There are several places in the error message that refer to a pattern with no name!

                   in an equation for ‘pattern’

        the inferred type of pattern :: Foo a -> p at <interactive>:1:1-21

    • Relevant bindings include
        pattern :: Foo a -> p (bound at <interactive>:1:1)

1. Why is this error happening in the first place? The error message mentions the type Foo a -> p, but in pattern Bar = Nothing, there isn't anything that should touch Foo.

Note that this bug does not occur if a slightly different (but ostensibly equivalent) type signature for Bar is given in the original source file:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
module Works where

data Foo a where
  FooInt :: Foo Int

pattern Bar :: Foo Int
pattern Bar = FooInt

λ> pattern Bar = Nothing
λ> :i Bar
pattern Bar :: Foo Int

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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