Associated type family: panic due to mismatch in arity of default instances

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:

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:

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:

class C a where
  type D a :: *
  type instance D a x = x

but increasing the arity of the family's kind

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

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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