[GHC] #15278: Add -Werror=compat, enable it in the testsuite

[GHC]

#15278: Add -Werror=compat, enable it in the testsuite
-------------------------------------+-------------------------------------
        Reporter:  int-index         |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"165d3d5ddaecc7dbe7f5ac051834a7619463efb0/ghc" 165d3d5/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="165d3d5ddaecc7dbe7f5ac051834a7619463efb0"
 Enable -Wcompat=error in the testsuite

 Enabling -Werror=compat in the testsuite allows us to easily see the
 impact that a new warning has on code. It also means that in the period
 between adding the warning and making the actual breaking change, all
 new test cases that are being added to the testsuite will be
 forwards-compatible. This is good because it will make the actual
 breaking change contain less irrelevant testsuite updates.

 Things that -Wcompat warns about are things that are going to break in
 the future, so we can be proactive and keep our testsuite
 forwards-compatible.

 This patch consists of two main changes:

 * Add `TEST_HC_OPTS += -Werror=compat` to the testsuite configuration.
 * Fix all broken test cases.

 Test Plan: Validate

 Reviewers: hvr, goldfire, bgamari, simonpj, RyanGlScott

 Reviewed By: goldfire, RyanGlScott

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15278

 Differential Revision: https://phabricator.haskell.org/D5200
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15278#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler