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  • Glasgow Haskell Compiler
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  • #12819
Closed
Open
Created Nov 10, 2016 by Richard Eisenberg@raeDeveloper

Pattern synonym signatures are not validity-checked

Ponder this garbage:

{-# LANGUAGE PatternSynonyms, ViewPatterns,
             TypeFamilies, KindSignatures #-}

module Bug where

type family F a  -- F :: * -> *
data T :: (* -> *) -> *

pattern Q :: T F -> String
pattern Q x <- (undefined -> x)

This is accepted by GHC 8.0.1 and HEAD. But it has an unsaturated type family! Urk!

The problem is that TcSigs.tcPatSynSig never does validity checking.

Trac metadata
Trac field Value
Version 8.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
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