[GHC] #14419: Check kinds for ambiguity

[GHC]

#14419: Check kinds for ambiguity
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType
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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Description changed by goldfire:

Old description:

> GHC does an ambiguity check on types. It should also do the same for
> kinds. Here is a program that should be rejected:
>
> {{{#!hs
> type family F a
> data T :: F a -> Type
> }}}
>
> `T`'s kind is ambiguous, and any occurrence of `T` will be rejected.
> Instead of rejecting usage sites, let's just reject the definition site.
>
> This check would be disabled by `AllowAmbiguousTypes`.
>
> Happily, I think the implementation should be easy, and that the current
> algorithm to check for ambiguous types should work for kinds, too. After
> all, types and kinds are the same these days.
>
> This was inspired by #14203, but no need to read that ticket to
> understand this one.

New description:

 GHC does an ambiguity check on types. It should also do the same for
 kinds. Here is a program that should be rejected:

 {{{#!hs
 type family F a
 data T :: F a -> Type
 }}}

 `T`'s kind is ambiguous, and any occurrence of `T` will be rejected.
 Instead of rejecting usage sites, let's just reject the definition site.

 This check would be disabled by `AllowAmbiguousTypes`.

 Happily, I think the implementation should be easy, and that the current
 algorithm to check for ambiguous types should work for kinds, too. After
 all, types and kinds are the same these days.

 This was inspired by #14203, but no need to read that ticket to understand
 this one.

 EDIT: See comment:8 and comment:10 for the nub of what needs to be done
 here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14419#comment:11>
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