[GHC] #15557: Reduce type families in equations' RHS when testing equation compatibility

[GHC]

#15557: Reduce type families in equations' RHS when testing equation compatibility
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        Reporter:  mniip             |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8423 #15546      |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > That doesn't happen:

 I think you mean more than `:t` reports the wrong result.  Consider
 {{{
 f :: Proxy b -> Proxy b -> Int
 f = error "urk"

 g :: Proxy a -> Proxy (Test a a) -> Int
 g x y = f x y
 }}}
 This indeed fails with
 {{{
 T15557.hs:25:13: error:
     • Couldn't match type ‘a’ with ‘Test a a’
 }}}
 I'd need to look back carefully at the paper to see what should happen,
 but no time for that today. Richard may opine on his return.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15557#comment:1>
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