[GHC] #15322: `KnownNat` does not imply `Typeable` any more when used with plugin

[GHC]

#15322: `KnownNat` does not imply `Typeable` any more when used with plugin
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        Reporter:  chshersh          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  typeable,knownnat
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10348            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by diatchki):

 My thinking was as follows: we are treating `Nat` and `Symbol` as just
 infinite families of type constructors.  This makes is tricky to define
 class instances involving them, as we essentially need to write infinitely
 many instances.  For example:

 {{{
 instance MyClass (T 0) where ...
 instance MyClass (T 1) where ...
 ...etc...
 }}}

 To avoid having to provide some special mechanism for declaring such
 instances, we just define one special class that captures this pattern,
 and the instances are "magically" provided by GHC.  For `Nat` it is called
 `KnownNat`:

 {{{
 instance KnownNat 0 where ...
 instance KnownNat 1 where ...
 ... etc ..
 }}}

 This makes it possible to define "infinite" instances for any class like
 this:

 {{{
 instance KnownNat n => MyClass (T n) where ...
 }}}

 We do the exact same thing for `Typeable`:
 {{{
 instance KnownNat n => Typeable (n :: Nat) where ...
 }}}

 The `KnownNat` constraint provides the run-time representation of the
 number, which is used to build the run-time representation of the type.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15322#comment:3>
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