[GHC] #14609: Per-instance UndecidableInstances

[GHC]

#14609: Per-instance UndecidableInstances
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        Reporter:  ryanreich         |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      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:                    |
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Comment (by AntC):

 Ooh-er. I've just seen that Type Families also use the
 `UndecidableInstances` flag.
 https://downloads.haskell.org/~ghc/8.2.2/docs/html/users_guide/glasgow_exts.html
 #type-family-decidability

 The whole idea of making the flag per-instance is that we avoid putting a
 blanket flag on a whole module. So implementing this ticket needs a per-
 type-family-instance pragma as well as a per-class-instance(?). Then for
 Closed Type Families: is that a pragma for the whole family decl, or per-
 equation? yeuch.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14609#comment:3>
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