[GHC] #13154: Standalone-derived anyclass instances aren't as permissive as empty instances

[GHC]

#13154: Standalone-derived anyclass instances aren't as permissive as empty
instances
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4337,
       Wiki Page:                    |  Phab:D4370
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Comment (by RyanGlScott):

 No. There is a smattering of things that still aren't supported, which
 include (but may not be limited to):

 * Instances for bare type variables:

 {{{#!hs
 instance Foo a -- Works
 deriving anyclass instance Foo a -- Doesn't work
 }}}

 * Instances for type-level literals:

 {{{#!hs
 instance Bar 1 -- Works
 deriving anyclass instance Bar 2 -- Doesn't work
 }}}

 * Empty instances:

 {{{#!hs
 instance Baz -- Works
 deriving instance Baz -- Doesn't work
 }}}

 I have an idea of how to fix these, but haven't had the time to try it out
 yet.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13154#comment:18>
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