Strange error message for deriving Data

The following code

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}

import Data.Data

data Pass = Parsed | Renamed | Typechecked
         deriving (Data)

data GHC (c :: Pass)

deriving instance Data (GHC 'Parsed)
deriving instance Data (GHC 'Renamed)
deriving instance Data (GHC 'Typechecked)

-- Type synonyms as a shorthand for tagging
type GhcPs  = GHC 'Parsed
type GhcRn  = GHC 'Renamed
type GhcTc  = GHC 'Typechecked

loads fine in ghci-8.2.0.20170507.

In ghc-8.0.2 and current master (as at https://github.com/ghc/ghc/tree/wip/new-tree-one-param-2) it gives

alanz@alanz-laptop:~/tmp/ghc-bug-data-deriving$ ghci-8.0.2 Main.hs 
GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/alanz/.ghci
[1 of 1] Compiling Main             ( Main.hs, interpreted )

Main.hs:16:1: error:
    Multiple declarations of ‘$tFl4AgeEJ0Vk5ONrnm56S3r’
    Declared at: Main.hs:16:1
                 Main.hs:17:1

Main.hs:16:1: error:
    Multiple declarations of ‘$tFl4AgeEJ0Vk5ONrnm56S3r’
    Declared at: Main.hs:15:1
                 Main.hs:17:1

Main.hs:16:1: error:
    Duplicate type signatures for ‘$tFl4AgeEJ0Vk5ONrnm56S3r’
    at Main.hs:15:1-36
       Main.hs:16:1-37
       Main.hs:17:1-41
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

I'm not sure what changes you did on your local branch to make this fail, but on GHC HEAD (as of commit 09d5c993), that program typechecks.

Do you have commit 895eefa8 (Make unique auxiliary function names in deriving) in your local branch? That's what fixed #12245 (closed), which I would consider this ticket to be a duplicate of.

Trac metadata

Trac field	Value
Related	- → #12245 (closed)

alanz helped me figure out what was going on. You won't hit the bug with the original code, but you will with this slightly tweaked code:

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE FlexibleInstances #-}

import Data.Data

data Pass = Parsed | Renamed | Typechecked
         deriving (Data)

data GHC (c :: Pass)

deriving instance Data GhcPs
deriving instance Data GhcRn
deriving instance Data GhcTc

-- Type synonyms as a shorthand for tagging
type GhcPs  = GHC 'Parsed
type GhcRn  = GHC 'Renamed
type GhcTc  = GHC 'Typechecked

Crucially, we're deriving instances for the type synonyms.

closed

Please ignore what I wrote in ticket:13759#comment:137438, that is utter garbage. The real, //real// cause of what was going on is much simpler: it is indeed a duplicate of #12245 (closed). The reason that alanz hit this bug when building ​https://github.com/ghc/ghc/tree/wip/new-tree-one-param-2 was simply because he was bootstrapping with GHC 8.0.2, and the fix for #12245 (closed) didn't land until GHC 8.2.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

added deriving label

added Pnormal label