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Let's say we have the following setup:

{-# LANGUAGE GADTs, PatternSynonyms, ViewPatterns #-}

data T a where
  MkT1 :: a -> T a
  MkT2 :: a -> T (Maybe a)

f :: (Eq a) => a -> a
f = id

Then the following definition works as expected:

pattern P1 x <- MkT1 (f -> x)

with the following inferred type:

λ» :i P1
pattern P1 :: () => Eq t => t -> T t

However, trying to do the same with {{MkT2}} doesn't work:

pattern P2 x <- MkT2 (f -> x)

this results in

Could not deduce (Eq a1) arising from a use of ‘f’
from the context (t ~ Maybe a1)
  bound by a pattern with constructor
             MkT2 :: forall a. a -> T (Maybe a),
           in a pattern synonym declaration

Trac metadata

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