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Summary

When a pattern synonym is bundled with a type, the type of the pattern synonym is required to match. This can be circumvented by giving the pattern a type annotation in which the head is polymorphic, but the context constrains it to be a type other than what it is being bundled with.

I'm afraid to say that code taking advantage of this bug does already exist, I'm currently using it as a means to allow importing several pattern synonyms at once as a workaround for https://github.com/ghc-proposals/ghc-proposals/pull/28

The users guide states

Bundled pattern synonyms are type checked to ensure that they are of the same type as the type constructor which they are bundled with. A pattern synonym P can not be bundled with a type constructor T if P‘s type is visibly incompatible with T.

The users guide also mentions that the types in the example below are distinct for pattern synonyms. https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#typing-of-pattern-synonyms

Steps to reproduce

• Compile this module

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE EmptyDataDecls #-}

module A
  ( Foo
  , FooPatterns(One) -- No error is generated here
  ) where

newtype Foo = Foo Int

-- | An empty data declaration used to export the "One" pattern
data FooPatterns

-- The commented signature causes GHC to fail with `Couldn't match expected
-- type of ‘FooPatterns’ with actual type of ‘Foo’` when exporting this pattern
-- bundled with `FooPatterns above`.

-- pattern One :: Foo
pattern One :: (a ~ Foo) => a
pattern One = Foo 1

• Observe that no error is generated.

• Remove the type annotation for pattern One

• Observe that GHC reports the following error:

A.hs:7:5-20: error:
    • Pattern synonyms can only be bundled with matching type constructors
      Couldn't match expected type of ‘FooPatterns’ with actual type of ‘Foo’
    • In the pattern synonym: One
      In the export: FooPatterns(One)
  |
7 |   , FooPatterns(One)
  |     ^^^^^^^^^^^^^^^^

Expected behavior

GHC reports the error in both cases

Environment

This behavior is present in GHC versions 8.7.20190115, 8.6.4, 8.4.4 and 8.2.2 at least.