[GHC] #16018: Disabling core optimizations ignores code that would otherwise warn.

[GHC]

#16018: Disabling core optimizations ignores code that would otherwise warn.
-------------------------------------+-------------------------------------
        Reporter:  dmjio             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.2
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  core,
                                     |  optimizations, Wall
Operating System:  Linux             |         Architecture:  x86
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Old description:

> Given the below code snippet,
>
> {{{#!hs
> module Main where
>
> main :: IO ()
> main = putStrLn "hey"
>
> data StopLight
>   = Red
>   | Green
>   | Yellow
>   deriving (Show, Eq)
>
> data Intersection
>   = Intersection
>   { light :: {-# UNPACK #-} !StopLight
>   } deriving (Show, Eq)
> }}}
>
> The above code will warn with optimizations enabled (`ghc -Wall -O2
> Main.hs`), message below:
> '''
> Main.hs:13:5: warning:
>     • Ignoring unusable UNPACK pragma
>         on the first argument of ‘Intersection’
>     • In the definition of data constructor ‘Intersection’
>       In the data type declaration for ‘Intersection’
>    |
> 13 |   = Intersection
> '''
>
> Without optimizations, no warnings are emitted `ghc -Wall -O0 Main.hs`.
>
> The desired behavior should be a warning emitted in both cases.

New description:

 Given the below code snippet,

 {{{#!hs
 module Main where

 main :: IO ()
 main = putStrLn "hey"

 data StopLight
   = Red
   | Green
   | Yellow
   deriving (Show, Eq)

 data Intersection
   = Intersection
   { light :: {-# UNPACK #-} !StopLight
   } deriving (Show, Eq)
 }}}

 The above code will warn with optimizations enabled (`ghc -Wall -O2
 Main.hs`), message below:
 {{{
 Main.hs:13:5: warning:
     • Ignoring unusable UNPACK pragma
         on the first argument of ‘Intersection’
     • In the definition of data constructor ‘Intersection’
       In the data type declaration for ‘Intersection’
    |
 13 |   = Intersection
 }}}

 Without optimizations, no warnings are emitted `ghc -Wall -O0 Main.hs`.

 The desired behavior should be a warning emitted in both cases.

--

Comment (by bgamari):

 I believe this may be similar to #9370. In short, whether we represent
 field unpackedness (and strictness) in the AST is determined by whether we
 have compiled with `-O`. More concretely, we drop the unpack pragma in
 `MkId.dataConSrcToImplBang` in the case of `-O0`.

 This is really an awful design and should be changed. As described in
 #9370, we should rather always include these sorts of pragmas in the AST
 and ignore them in the simplifier if optimisation is disabled.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16018#comment:2>
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