Panic "tcTyVarDetails" in GHCi when :forcing a data family instance

[goldfire]

I loaded the following code into GHCi:

{-# LANGUAGE GADTs, DataKinds, KindSignatures, TypeFamilies, PolyKinds #-}

data Nat = Zero | Succ Nat
data family Sing (a :: k)
data instance Sing (a :: Nat) where
  SZero :: Sing Zero
  SSucc :: Sing n -> Sing (Succ n)

Then. I run these commands at the prompt:

*Main> let x = SSucc SZero
*Main> :f x

The response is

ghc: panic! (the 'impossible' happened)
  (GHC version 7.7.20121031 for x86_64-apple-darwin):
	tcTyVarDetails
<<details unavailable>>

[simonpj@…]

commit acbe5265d053e6497626f12f49f38aa31ff730e5

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Tue Nov 6 15:55:39 2012 +0000

    Fix the instantiation of data constructors in the GHCi debugger
    
    This bug caused Trac #7386, because in the (rather tricky) "type
    inference" (aka run time type reconstruction) done by the GHCi
    debugger, we were failing to instantiate a data type family
    correctly.  When that code was written we didn't *have* data
    families.
    
    I wrote Note [Constructor arg types] to explain the new scheme.

 compiler/ghci/RtClosureInspect.hs |   39 ++++++++++++++++++++++++++++++++----
 1 files changed, 34 insertions(+), 5 deletions(-)

[simonpj]

Thanks; I've fixed this. Merge to 7.6 branch.

Simon

[igloo]

Merged as c41ca7980664c73cfbd27b0d9f9c081ecf7dc25c