Unboxed tuples as values

[simonpj]

Consider

  type T a = Int -> (# Int, Int #)

  f :: T a -> T a
  f t = \x -> case t x of r -> r

gives the following error message:

  ghc-5.04.1: panic! (the `impossible' happened, GHC 
version 5.04.1):
	  codeGen/CgRetConv.lhs:83: Non-exhaustive 
patterns in function
	  dataReturnConvPrim

Trouble is that 'r' is bound to an unboxed tuple.  This 
should fail, but in 5.04.1 it crashes the compiler.  In 6.0 
it's worse: the code will core dump because it enters R1.

But it should be rejected in the type checker.

[simonpj]

Logged In: YES 
user_id=50165

Fixed, at last. Test is tcfail115.hs

The error message is still poor, but at least it now fails 
typechecking