[GHC] #16220: -Wmissing-signatures should warn when top-level signature is partial

[GHC]

#16220: -Wmissing-signatures should warn when top-level signature is partial
-------------------------------------+-------------------------------------
           Reporter:  aspiwack       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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:
-------------------------------------+-------------------------------------
 I'd like GHC to emit a warning for the following definition:

 {{{#!hs
 g :: _
 g = True
 }}}

 But not for that one:

 {{{#!hs
 f :: Int
 f = x
   where
     x :: _
     x = 1
 }}}

 The context is that I'm using partial signatures to help the type checker
 give the right type to a binding, with no intention of filling the holes
 later. (what Richard calls elision [https://github.com/ghc-proposals/ghc-
 proposals/pull/194 here])

 The warning accompanying partial signatures (`-Wpartial-type-signatures`)
 effectively treats `_`-s in type signatures as holes which are meant to be
 filled (the type-level equivalent of typed holes). Which is not useful to
 my usage.

 I can deactivate `-Wpartial-type-signatures`, however, it seems to me
 that, in that case, a partial top-level signature is just as bad as a
 missing top-level signature. So it seems to me that whenever I have
 `-Wmissing-signatures` enabled I also want to warn for partial top-level
 signatures.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16220>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler