[GHC] #10079: Coercible solver regression: Couldn't match rep of () with Const () b

[GHC]

#10079: Coercible solver regression: Couldn't match rep of () with Const () b
-------------------------------------+-------------------------------------
        Reporter:  glguy             |                   Owner:  goldfire
            Type:  bug               |                  Status:  merge
        Priority:  highest           |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc1
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:  indexed-
  valid program                      |  types/should_compile/T10079
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 I'm not sure this is right yet.

 '''First'''

  * For some reason `can_eq_nc'` only calls `can_eq_wanted_app` in the
 wanted/derived case.

  * That means that `try_decompose_app` cannot assume that its arg types
 are flattened.  But `try_decompose_nom_app` and `try_decompose_repr_app`
 seem to assume that the type ''is'' flattened.

 The simple thing would be to rename `can_eq_wanted_app` to `can_eq_app`,
 and call it in all cases.

 '''Second'''.  In your new `try_decompose_repr_app` you fail if either is
 an `AppTy`.  But what about
 {{{
   f a ~R g f a
 }}}
 where `g` flattens to `N`, a newtype
 {{{
 newtype N f a = MkN (f a)
 }}}
 Then the flattened version will be
 {{{
   f a ~R N f a
 }}}
 and that is certainly soluble.

 '''Third'''.  What about
 {{{
    a ~ f a
 }}}
 where `f` flattens to `N2` and
 {{{
 newtype N2 a = MkN2 a
 }}}
 At the moment we will miss this altogether.  We want an invariant that a
 representational `CTyEqCan` has a right-hand side that is not a newtype
 application.  And we need to ensure that the invariant holds.

 So this really doesn't look good yet.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10079#comment:9>
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