Incomplete-pattern checking for n+k patterns is not implemented

[simonpj]

The check for incomplete patterns does not understand about n+k patterns. Here's an example:

f 0     = True
f (n+1) = False

GHC says, rather confusingly:

    Warning: Pattern match(es) are non-exhaustive
	     In the definition of `f':
		 Patterns not matched: #x with #x `notElem` [0#]

The reason is in compiler/deSugar/Check.hs which says

	-- Horrible hack.  The simplify_pat stuff converts NPlusK pats to WildPats
	-- which of course loses the info that they can fail to match.  So we 
	-- stick in a CanFail as if it were a guard.
	-- The Right Thing to do is for the whole system to treat NPlusK pats properly

See ticket:595

[simonpj]

See Task #595

[igloo]

The !NoNPlusKPatterns proposal was accepted for H'2010, so I don't think it is worth spending time on this.