[GHC] #15584: nonVoid is too conservative w.r.t. strict argument types

[GHC]

#15584: nonVoid is too conservative w.r.t. strict argument types
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15305            |  Differential Rev(s):  Phab:D5116
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"e68b439fe5de61b9a2ca51af472185c62ccb8b46/ghc" e68b439/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e68b439fe5de61b9a2ca51af472185c62ccb8b46"
 Add a recursivity check in nonVoid

 Summary:
 Previously `nonVoid` outright refused to call itself
 recursively to avoid the risk of hitting infinite loops when
 checking recurisve types. But this is too conservative—we //can//
 call `nonVoid` recursively as long as we incorporate a way to detect
 the presence of recursive types, and bail out if we do detect one.
 Happily, such a mechanism already exists in the form of `checkRecTc`,
 so let's use it.

 Test Plan: make test TEST=T15584

 Reviewers: simonpj, bgamari

 Reviewed By: simonpj

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15584

 Differential Revision: https://phabricator.haskell.org/D5116
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15584#comment:4>
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