[GHC] #9562: Type families + hs-boot files = unsafeCoerce

[GHC]

#9562: Type families + hs-boot files = unsafeCoerce
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        Reporter:  goldfire          |                   Owner:  ezyang
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #10270            |  Differential Revisions:
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Comment (by ezyang):

 Here is something curious:

 {{{
 ezyang at sabre:~/Dev/haskell/T9562/p$ ghc --make D -fforce-recomp
 [1 of 5] Compiling A                ( A.hs, A.o )
 [2 of 5] Compiling B[boot]          ( B.hs-boot, B.o-boot )
 [3 of 5] Compiling C                ( C.hs, C.o )
 [4 of 5] Compiling B                ( B.hs, B.o )
 [5 of 5] Compiling D                ( D.hs, D.o )

 B.hs:8:15:
     Conflicting family instance declarations:
       type instance F a b -- Defined at B.hs:8:15
       type instance F a b -- Defined at D.hs:8:15
 }}}

 however,

 {{{
 ezyang at sabre:~/Dev/haskell/T9562/p$ ghc --make B C D A -fforce-recomp
 [1 of 5] Compiling A                ( A.hs, A.o )
 [2 of 5] Compiling B[boot]          ( B.hs-boot, B.o-boot )
 [3 of 5] Compiling C                ( C.hs, C.o )
 [4 of 5] Compiling D                ( D.hs, D.o )
 [5 of 5] Compiling B                ( B.hs, B.o )
 }}}

 I don't know why in the latter case no error is reported.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9562#comment:10>
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