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This is a really bizarre one. If you have the following program:

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}
module Main where

class LL f where
  go :: f a -> ()

instance LL [] where
  go _ = ()

pattern GoLL :: LL f => f a
pattern GoLL <- (go -> ())

{-# COMPLETE GoLL :: [] #-}

goLLEx :: [a] -> Int
goLLEx GoLL = 5

main :: IO ()
main = return ()

And you compile it with GHC HEAD in just the right way, GHC hangs forever!

$ ~/Software/ghc3/inplace/bin/ghc-stage2 -no-link Bug.hs
[1 of 1] Compiling Main             ( Bug.hs, Bug.o )
$ ~/Software/ghc3/inplace/bin/ghc-stage2 Bug.hs
# Hangs forever here

That -no-link part is crucial, since if you compile it straight from a source file, it works. Also worth noting:

• I can reproduce this in GHC HEAD, but not in GHC 8.0.2.
• The COMPLETE pragma is critical for triggering this bug. If you comment it out, then the issue disappears.

Note that cabal-install installs executables using this -no-link strategy, so this bug effectively prevents me from using cabal-install with code that involves COMPLETE pragmas. Marking as high priority.

I have //no// idea where to start looking for this one. Matthew, do you have any idea what might be happening here?

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mpickering
Operating system
Architecture