[GHC] #14362: Allow: Coercing (a:~:b) to (b:~:a)

[GHC]

#14362: Allow: Coercing (a:~:b) to (b:~:a)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  roles
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 But what is the most general type of `convert Refl = Refl`?

 We cannot write

 {{{#!hs
 -- Could not deduce: a' ~ b'
 -- from the context: a  ~ b
 convert :: (a :~: b) -> (a' :~: b')
 convert Refl = Refl
 }}}

 note how type checking this depends on the ‘underlying constraint’ of
 `(:~:)`: `(~)`.

 We have to be deduce `a' ~ b'` from the given context `a ~ b`, which is
 this?

 {{{#!hs
 convert :: ((a~b) => (a'~b')) => (a :~: b) -> (a' :~: b')
 convert Refl = Refl
 }}}

 so I would expect `coerce` to have that type.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14362#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler