[GHC] #15398: GADT deriving Ord generates inaccessible code in a pattern with constructor.

[GHC]

#15398: GADT deriving Ord generates inaccessible code in a pattern with
constructor.
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        Reporter:  philderbeast      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  deriving,
                                     |  PatternMatchWarnings
Operating System:  MacOS X           |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > I thought you were requesting that we never emit inaccessible code
 warnings for derived code (regardless of whether -Winaccessible-code is
 enabled or not).

 Correct.  So if we switch off `-Winaccessible-code` when typechecking
 derived code, that would do job.  And we try to; see
 {{{
                 -- Now GHC-generated derived bindings, generics, and
 selectors
                 -- Do not generate warnings from compiler-generated code;
                 -- hence the use of discardWarnings
         tc_envs@(tcg_env, tcl_env)
             <- discardWarnings (tcTopBinds deriv_binds deriv_sigs) ;
 }}}
 in `TcRnDriver`. Unfortunately the `dicardWarnings` only discards warnings
 generated while doing `tcTopBinds`.  But `tcTopBinds` also creates a bunch
 of constraints that don't "see" that `discardWarnings`.

 If instead switched off the warning `DynFlags`, and captured the
 `DynFlags` in an implication, the warning suppression would still work on
 the constraints.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15398#comment:9>
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