Panic from combining generalized list comprehensions and record wildcards

Combining record wild cards with generalized list comprehension grouping causes the type-checker to mis-infer the type of the list comprehension, potentially leading to a panic. For example

data Rec = Rec {a :: Int} deriving (Show)

recs :: [[Int]]
recs = [a | Rec {..} <- [Rec 1], then group by a]

results in a type error, because GHC thinks recs has type [Int]. On the other hand,

recs :: [[Int]]
recs = [a | Rec {a=a} <- [Rec 1], then group by a]

compiles, correctly.

If the type annotation is omitted from the first example, GHC panics with

ghc: panic! (the 'impossible' happened)
  (GHC version 6.12.1 for x86_64-unknown-linux):
	expectJust rnStmt

I've attached a test file that demonstrates the bug.

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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