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When trying to give a type signature with a free type variable to a pattern in a pattern synonym, GHC dies with an internal error. Given the file

{-# LANGUAGE PatternSynonyms, ScopedTypeVariables #-}
pattern Nil = [] :: [a]

the resulting error is

Example.hs:2:22: GHC internal error: ‘a’ is not in scope during type checking, but it passed the renamer …
   tcl_env of environment: []
   In an expression type signature: [a]
   In the expression: [] :: [a]
   In an equation for ‘$WNil’: $WNil = [] :: [a]
Compilation failed.

The fix is to add RankNTypes and an explicit forall:

{-# LANGUAGE PatternSynonyms, ScopedTypeVariables, RankNTypes #-}
pattern Nil = [] :: forall a. [a]

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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