Panic when pattern matching record syntax constructor using wrong parameter

[samlich]

The following occurs

[1/1] Compiling Haskell                  Main                   (Test.hs)
ehc: panic: DataFieldPatExpr.Ext.ioffset

when compiling this (C is incorrectly pattern matched with y in the let)

module Main (main) where

data C = C {x :: Int}
data B = B {y :: Int}
main :: IO ()
main = let (C {y=z}) = C {x=5}
       in return ()

Using a symbol that doesn't exist (e.g. C {w=z}) works as expected.