[GHC] #11136: Associated type family: panic due to mismatch in arity of default instances

[GHC]

#11136: Associated type family: panic due to mismatch in arity of default instances
-------------------------------------+-------------------------------------
           Reporter:  airini         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Having extra type parameters in the definition of default instances for an
 associated type family causes a ghc panic error (should be detected as a
 not valid instance). Smallest case example:

 {{{#!hs
 class C a where
   type D a
   type instance D a x = x
 }}}

 It produces the error:

 {{{
 [1 of 1] Compiling Main             ( SplitKindFun.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.2 for x86_64-apple-darwin):
         splitKindFunTysN 1 *

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Crashes similarly for additional parameters in the default instance:

 {{{#!hs
 class C a where
   type D a
   type instance D a x y = x
 }}}

 {{{
 [1 of 1] Compiling Main             ( SplitKindFun.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.2 for x86_64-apple-darwin):
         splitKindFunTysN 2 *

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Presumably the arity is not compared correctly to that of the family
 declaration when the kind is default `*` since the following (and
 equivalent) causes the panic crash:

 {{{#!hs
 class C a where
   type D a :: *
   type instance D a x = x
 }}}

 but increasing the arity of the family's kind

 {{{#!hs
 class C a where
   type D a :: * -> *
   type instance D a x = x
 }}}

 raises the appropriate error:

 {{{
 [1 of 1] Compiling Main             ( SplitKindFun.hs, interpreted )

 SplitKindFun.hs:6:3:
     Number of parameters must match family declaration; expected 1
     In the default type instance declaration for ‘D’
     In the class declaration for ‘C’
 Failed, modules loaded: none.
 }}}

 Longer verbose compiler output of the original error:

 {{{
 $ ghc -v SplitKindFun.hs
 Glasgow Haskell Compiler, Version 7.10.2, stage 2 booted by GHC version
 7.10.1.20150612
 Using binary package database:
 /Library/Frameworks/GHC.framework/Versions/7.10.2-x86_64/usr/lib/ghc-7.10.2/package.conf.d/package.cache
 Using binary package database:
 /Users/uu/.ghc/x86_64-darwin-7.10.2/package.conf.d/package.cache
 wired-in package ghc-prim mapped to ghc-
 prim-0.4.0.0-af16264bc80979d06e37ac63e3ba9a21
 wired-in package integer-gmp mapped to integer-
 gmp-1.0.0.0-8e0f14d0262184533b417ca1f8b44482
 wired-in package base mapped to
 base-4.8.1.0-075aa0db10075facc5aaa59a7991ca2f
 wired-in package rts mapped to builtin_rts
 wired-in package template-haskell mapped to template-
 haskell-2.10.0.0-161ca39a5ae657ff216d049e722e60ea
 wired-in package ghc mapped to ghc-7.10.2-5c2381785a7b22838c6eda985bc898cf
 wired-in package dph-seq not found.
 wired-in package dph-par not found.
 Hsc static flags:
 wired-in package ghc-prim mapped to ghc-
 prim-0.4.0.0-af16264bc80979d06e37ac63e3ba9a21
 wired-in package integer-gmp mapped to integer-
 gmp-1.0.0.0-8e0f14d0262184533b417ca1f8b44482
 wired-in package base mapped to
 base-4.8.1.0-075aa0db10075facc5aaa59a7991ca2f
 wired-in package rts mapped to builtin_rts
 wired-in package template-haskell mapped to template-
 haskell-2.10.0.0-161ca39a5ae657ff216d049e722e60ea
 wired-in package ghc mapped to ghc-7.10.2-5c2381785a7b22838c6eda985bc898cf
 wired-in package dph-seq not found.
 wired-in package dph-par not found.
 *** Chasing dependencies:
 Chasing modules from: *SplitKindFun.hs
 Stable obj: []
 Stable BCO: []
 Ready for upsweep
   [NONREC
       ModSummary {
          ms_hs_date = 2015-11-26 15:20:15 UTC
          ms_mod = Main,
          ms_textual_imps = [import (implicit) Prelude]
          ms_srcimps = []
       }]
 *** Deleting temp files:
 Deleting:
 compile: input file SplitKindFun.hs
 Created temporary directory:
 /var/folders/m3/cx6_zbks6qq2xtwx92njp8w40000gn/T/ghc755_0
 *** Checking old interface for Main:
 [1 of 1] Compiling Main             ( SplitKindFun.hs, SplitKindFun.o )
 *** Parser:
 *** Renamer/typechecker:
 *** Deleting temp files:
 Deleting:
 /var/folders/m3/cx6_zbks6qq2xtwx92njp8w40000gn/T/ghc755_0/ghc_1.s
 Warning: deleting non-existent
 /var/folders/m3/cx6_zbks6qq2xtwx92njp8w40000gn/T/ghc755_0/ghc_1.s
 *** Deleting temp dirs:
 Deleting: /var/folders/m3/cx6_zbks6qq2xtwx92njp8w40000gn/T/ghc755_0
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.2 for x86_64-apple-darwin):
         splitKindFunTysN 1 *

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11136>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler