[project @ 2001-08-21 13:21:39 by simonpj]

[project @ 2001-08-21 13:21:39 by simonpj]
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	Fix a tyvar scoping bug
	-----------------------

This program:

	data T = forall a. T a (a->Int)

	f :: T -> T
	f (T (x::a) f) = T (undefined::a) f

gave the error

    Inferred type is less polymorphic than expected
    Quantified type variable `a' is unified with another quantified type variable `a'
    When checking a pattern that binds f :: a -> Int
    In the definition of `f': f (T (x :: a) f) = T (undefined :: a) f

This is of course bogus.  The fix is in TcMatches.tcMatchPats, where the
in-scope tyvars should be un-extended before calling tcCheckExistentialPat.