Fix Trac #3219: type of a record update
Record updates are amazingly hard to typecheck right. This is one place where GHC's policy of typechecking the original source is much harder than desugaring and typechecking that! Anyway, the bug here is that to compute the 'fixed' type variables I was only looking at one constructor rather than all the relevant_cons Test is typecheck/should_compile/T3219, which GHC 6.10 barfs on (with Lint).
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