[GHC] #16305: When should -Wmissed-specializations fire?

11 Feb 2019

      #16305: When should -Wmissed-specializations fire?
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.7
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While working on a fix for #16282, I noticed:

 1. that `-Wmissed-specializations` doesn't fire when the docs say it
 should
 2. that the "probable fix" suggested when a warning does fire is highly
 suspect

 {{{
 import Data.Map
 main :: IO ()
 main = do
   let m = [] :: [Map String Bool]
   mapM_ print m
 }}}

 With GHC 8.4.3, 8.6.3, and HEAD:
 `ghc -O2 -Wmissed-specializations Main.hs` does *not* issue any warnings.

 However, `ghc -O2 -Wall-missed-specializations Main.hs` *does* create a
 warning:
 {{{
 Main.hs: warning: [-Wall-missed-specialisations]
     Could not specialise imported function
 ‘Data.Map.Internal.$w$cshowsPrec’
       when specialising ‘Data.Map.Internal.$fShowMap_$cshowsPrec’
     Probable fix: add INLINABLE pragma on
 ‘Data.Map.Internal.$w$cshowsPrec’
 }}}

 The docs for `-Wmissed-specializations` say, "warn when specialisation of
 an imported, overloaded function fails." Since `showsPrec` is an imported
 function, it seems that a warning should have been issued with `-Wmissed-
 specs`. My reading of the docs is that `-Wall-missed-specs` should output
 everything `-Wmissed-specs` does, along with any *local* overloaded and
 unspecialized functions.

 Moreover, the "Probable fix" is suspect. A warning recommending an
 `INLINABLE` pragma is issued depending on the output of `warnMissedSpec`
 in specialise/Specialise.hs. For `-Wall-missed-specs`, `warnMissedSpec`
 doesn't check if an INLINABLE pragma is already present, so the fix could
 be redundant. For `-Wmissed-specs`, `warnMissedSpecs` *only* issues a
 warning if there *is* any inline pragma (of one sort of another) on all
 the callers, making the `probable fix` definitely redundant.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16305
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