PatternSynonyms + ScopedTypeVariables triggers an internal error
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 |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture |