[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
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Well we hvae
 {{{
 instance Show (Proxy s) where ...
 }}}
 Notice that `s` is not used.  So the instance will work just fine if `s` =
 `Head '[]`.

 I don't regard this as a bug (yet, anyway).  It accepts more programs than
 at least some people expect; but well typed programs (ones that are
 accepted) don't go wrong at runtime, which is what we ask from our type
 system.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13775#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler