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

[GHC]

#16305: When should -Wmissed-specializations fire?
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        Reporter:  crockeea          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             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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Comment (by simonpj):

 As I read the code it is trying to do this:

 * `-Wall-missed-specs`: warn about ''any'' overloaded imported functions
 that cannot be specialised.  Usually this means we don't have its RHS.

 * `-Wmissed-specs`: warn about any overloaded imported functions that
 can't be specialised, and is transitively called by a chain of imported
 functions each with an INLINEABLE pragma.

 For the latter consider
 {{{
 module A where
   import B
   foo = f (3::Int) + g (4::Int)

 module B where
   f :: Num a => a -> a
   {-# INLINABLE f #-}
   f x = h x

   g x = Num a => a -> a
   g x = blah

   h x :: Num a => a -> a
   h x = blah
 }}}
 Then when compiling A, `-Wmissed-specs` is supposed to

 * Not warn about not-specialising `g`, because `g` didn't have an
 INLINABLE pragma.

 * Warn about non-specialising `h` because `f` (which is directly called
 from A) has a careful `INLINABLE` pragma, but `h` (perhaps due to an
 oversight) does not.

 In contrast `-Wall-missed-specs` should warn about both.

 Does that make sense?  Is it what we want?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16305#comment:6>
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