[GHC] #14481: Mutually dependent modules with orphan instances causes missing symbols with single-shot compilation

17 Nov 2017

      #14481: Mutually dependent modules with orphan instances causes missing symbols
with single-shot compilation
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Alan Zimmerman encountered this nasty bug in his Trees That Grow branch:

 Consider we have the following types,
 {{{#!hs
 module Types where

 data Expr p = Var String
             | App (Expr p) (Expr p)
             | ADecl (Decl p)

 data Decl p = Bind String (Expr p)
 }}}

 Now imagine that for some reason we want to define orphan instances for
 some class (`Show`, for instance) for these types in two separate modules.
 We would have:
 {{{#!hs
 -- Instances1.hs
 module Instances1 where

 import {-# SOURCE #-} Instances2 ()
 import Types

 deriving instance Show (Decl p)

 -- Instances1.hs-boot
 module Instances1 where
 import Types
 instance Show (Decl p)

 -- Instances2.hs
 module Instances2 where

 import {-# SOURCE #-} Instances1 ()
 import Types

 deriving instance Show (Expr p)

 -- Instances2.hs-boot
 module Instances2 where
 import Types
 instance Show (Expr p)
 }}}

 Now, for instance, say we have some program that uses this whole mess,
 {{{#!hs
 -- Main.hs
 module Main where
 import Types

 -- Use SOURCE import to ensure GHC doesn't grab dictionary from unfolding
 in
 -- interface file
 import {-# SOURCE #-} Instances2

 main = putStrLn $ show $ (Var "hi" :: Expr Int)
 }}}

 With `--make` mode we can compile `Main.hs` with no trouble:
 {{{
 $ ghc --make Main.hs
 [1 of 6] Compiling Types            ( Types.hs, Types.o )
 [2 of 6] Compiling Instances2[boot] ( Instances2.hs-boot,
 Instances2.o-boot )
 [3 of 6] Compiling Main             ( Main.hs, Main.o )
 [4 of 6] Compiling Instances1[boot] ( Instances1.hs-boot,
 Instances1.o-boot )
 [5 of 6] Compiling Instances2       ( Instances2.hs, Instances2.o )
 [6 of 6] Compiling Instances1       ( Instances1.hs, Instances1.o )
 Linking Main ...
 $ ./Main
 Var "hi"
 }}}

 However, if we instead use single-shot mode, we end up never producing
 object code for  one of the boot DFuns,
 {{{
 $ ghc -c Types.hs
 $ ghc -c Instances1.hs-boot
 $ ghc -c Instances2.hs
 $ ghc -c Instances2.hs-boot
 $ ghc -c Instances1.hs
 $ ghc -c Main.hs
 $ ghc -o test Types.o Instances1.o Instances2.o Main.o
 Main.o:s1lN_info: error: undefined reference to
 'Instances2_zdfxShowExpr_closure'
 Main.o(.data.rel.ro+0x8): error: undefined reference to
 'Instances2_zdfxShowExpr_closure'
 collect2: error: ld returned 1 exit status
 `gcc' failed in phase `Linker'. (Exit code: 1)
 }}}

 In the case of `--make` mode the symbol in question is emitted in the
 object code for `Instances2`. However, when we use single-shot mode the
 `hi-boot` file for `Instances2` doesn't exist when the `hs` file is
 compiled. It seems that this makes the DFun impedance matching logic in
 `TcRnDriver.checkBootIface'` not fire.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14481
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