Non-exhaustive patterns in basicTypes/Var.lhs

[Peter Simons]

QuickCheck's 'generate' function works fine in GHCi, but
only for the _first_ time I call it. After that, I get an
error:

 | Ok, modules loaded: Main.
 | *Main> generate 3 (mkStdGen 28) (return 'x')
 | Loading package QuickCheck ... linking ... done.
 | 'x'
 |
 | *Main> generate 3 (mkStdGen 28) (return 'x')
 | *** Exception: basicTypes/Var.lhs:226:32-58: Non-exhaustive
 |     patterns in record update

Peter