[GHC] #15504: -XStrict doesn't prevent warnings from -Wunbanged-strict-patterns

10 Aug 2018


      #15504: -XStrict doesn't prevent warnings from -Wunbanged-strict-patterns
-------------------------------------+-------------------------------------
           Reporter:  ChaiTRex       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           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'm using:
 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 8.4.3
 }}}

 I was under the impression that
 [https://ghc.haskell.org/trac/ghc/wiki/StrictPragma#Strict -XStrict]
 automatically included outermost bang patterns, but either that isn't
 always the case or [https://downloads.haskell.org/~ghc/master/users-guide
 /using-warnings.html#ghc-flag--Wunbanged-strict-patterns -Wunbanged-
 strict-patterns] doesn't know that `-XStrict` did its job correctly:

 {{{#!hs
 {-# OPTIONS_GHC -Wunbanged-strict-patterns #-}
 {-# LANGUAGE BangPatterns, MagicHash, Strict, UnboxedTuples #-}

 module Example where

 import GHC.Exts  (Int(I#), quotRemInt#)

 lastDigit :: Int -> Int
 lastDigit (I# x) = let (# q, r #) = quotRemInt# x 10#
                    in  I# r
 }}}

 compiles with a warning:

 {{{
 [1 of 1] Compiling Example          ( Example.hs, Example.o )

 Example.hs:9:24: warning: [-Wunbanged-strict-patterns]
     Pattern bindings containing unlifted types should use
     an outermost bang pattern:
       (# q, r #) = quotRemInt# x 10#
   |
 9 | lastDigit (I# x) = let (# q, r #) = quotRemInt# x 10#
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15504
GHC http://www.haskell.org/ghc/
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