[GHC] #12564: Type family in type pattern kind

[GHC]

#12564: Type family in type pattern kind
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        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:  14119             |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 * Bizarrely, I don't think we need to do much proving. The type safety
 proof depends on the non-overlap of axiom LHSs. That's unchanged here --
 the overlap check must treat any type family application on an LHS just
 like it would a variable. The equality constraints serve only to reduce
 the applicability of axioms, never to increase it. Thus, no threats to
 safety.

 * But the evidence ''is'' used: it's passed to the coercion axiom. The
 evidence is the `coLen` in my example above.

 * I take back the "evidence may be mentioned" comment.

 * GHC passes loads of redundant information at every call site (e.g. `id
 @Bool True`). This is not new. The uniquely-determined check would help
 those other places too. I think, in the end, that idea is orthogonal to
 the plan here, which would work for `F x (G x) = True`, should we ever
 want to support that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12564#comment:15>
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