[GHC] #14838: missing "incomplete-patterns" warning for TH-generated functions

21 Feb 2018

      #14838: missing "incomplete-patterns" warning for TH-generated functions
-------------------------------------+-------------------------------------
           Reporter:  gelisam        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.2.2
  Haskell                            |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 "incomplete-patterns" warnings are generated for TH-generated case
 expressions, but not for TH-generated functions, so the behaviour is
 inconsistent.

 For example:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 module Lib where
 import Language.Haskell.TH

 qIncompleteCase :: Q [Dec]
 qIncompleteCase = [d|
   incompleteCase :: Bool -> ()
   incompleteCase b = case b of
     True -> () |]

 qIncompleteFunction :: Q [Dec]
 qIncompleteFunction =[d|
   incompleteFunction :: Bool -> ()
   incompleteFunction True = () |]
 }}}

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 module Bug where
 import Lib

 $qIncompleteCase

 $qIncompleteFunction

 incompleteCase' :: Bool -> ()
 incompleteCase' b = case b of
   True -> ()

 incompleteFunction' :: Bool -> ()
 incompleteFunction' True = ()
 }}}

 When compiling the above two files with `-Wall`, GHC 8.2.2 produces an
 "incomplete-patterns" warning for `qIncompleteCase`, `incompleteCase'`,
 and `incompleteFunction'`, but not for `qIncompleteFunction`. I would
 prefer to get a warning for `qIncompleteFunction` as well.

 My use case is the [https://hackage.haskell.org/package/surjective
 surjective] package, in which I intentionally generate code which produces
 warnings in order to warn the user about corresponding issues in their
 code. I could generate
 [https://github.com/gelisam/surjective/blob/master/src/Surjective.hs#L158-L16...
 better error messages] if GHC generated warnings for TH-generated
 functions as well.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14838
GHC http://www.haskell.org/ghc/
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