[GHC] #15379: Don't reject user-written instances of KnownNat and friends in hsig files

[GHC]

#15379: Don't reject user-written instances of KnownNat and friends in hsig files
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  backpack
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 simonpj):

 I'm not sure I'm following all the details here, but can you take
 inspiration from the way that instances in `hs-boot` files work?  If you
 have
 {{{
 Foo.hs-boot
   data T
   instance Eq T
 }}}
 then in the `hi-boot` file (which we get from compiling `Foo.hs-boot`) we
 will have something like
 {{{
 $d77 : Eq T
 }}}
 That is, the arbtrarily named `$d77` is a witness for `Eq T`.   Modules
 importing `Foo.hi-boot` use `$d77` when they need `Eq T`.

 Then when compiling `Foo.hs` we generate the real instance
 {{{
 $d98 = <the code for equality on T> :: Eq T
 }}}
 and (having read the `Foo.hi-boot` file, we also generate a compatibility
 stub
 {{{
 $d77= $d98
 }}}
 so that references to `$d77` will end up in `$d98`.

 Maybe the same approach will work for `hsig` files.  Edward will know.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15379#comment:5>
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