[GHC] #13775: Type family expansion is too lazy, allows accepting of ill-typed terms

[GHC]

#13775: Type family expansion is too lazy, allows accepting of ill-typed terms
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        Reporter:  fizruk            |                Owner:  diatchki
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |             Keywords:
      Resolution:                    |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by diatchki):

 I remember how the implementation works.

 My comment was about the fact that GHC is perfectly happy to treat "stuck"
 types as ordinary types without fully resolving them, which is why it can
 solve constraints such as `Show (Proxy (TypeError ...))`, or `Show (Proxy
 (F Int))`.

 A different design choice is to refuse to solve such constraints until GHC
 is sure that the type functions involved will produce a valid result, but
 that's not what we currently do.  As a result, neither the `TypeError` nor
 the missing `F Int` instance are reported, and the `Show` constraint is
 happily solved, which is---perhaps---somewhat confusing.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13775#comment:7>
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