When should -Wmissed-specializations fire?

While working on a fix for #16282 (closed), 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, in 8.6.3 and HEAD, 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.