[GHC] #13770: HEAD: Type mentioned in error won't show up in pattern signature

[GHC]

#13770: HEAD: Type mentioned in error won't show up in pattern signature
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        Reporter:  heisenbug         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Yes, this is a tidying problem.  The `PatSkol` constructor does not carry
 a tidy-able type.  It should be treated more like `SigSkol`.

 But what do we ''want'' the error message to say?  Consider
 {{{
 data T a where
   MkT :: a -> b -> T a

 f :: T [p] -> Int
 f (MkT x y) = ...
 }}}
 Now, if there is an error in the `...`, involving the skolem `b` bound by
 the `(MkT x y)` pattern, would we like this?
 {{{
       'b4' is a rigid type variable bound by
         a pattern with constructor:
            MkT :: [p] -> b4 -> T [p]
 }}}
 Notice that I have instantiated the universal argument too...  I see no
 alternative to doing so.  To be comprehensible we must instantiate the
 existential argument to match the one reported (remember there may be many
 nested `MkT` matches).

 That's what I propose. Sound ok?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13770#comment:3>
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