[GHC] #13006: Possible program should type check but does not using Implicit Parameters and Vectors

[GHC]

#13006: Possible program should type check but does not using Implicit Parameters
and Vectors
-------------------------------------+-------------------------------------
        Reporter:  clinton           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2-rc2
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 From the given `[G] ?v :: MVector (PrimState m) Int` and wanted `[G] ?v
 MVector (PrimState n) Int` we get the derived equality `[D] MVector
 (PrimState m) Int ~ MVector (PrimState n) Int`.

 Alas `PrimState` is a (non-injective) type family, so this does not tell
 us that `M ~ n`.  In the simpler version, it does.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13006#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler