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Simon Peyton Jones authored
When matching against a GADT, some of the constructors may be impossible. For example data T a where T1 :: T Int T2 :: T Bool T3 :: T a f :: T Int -> Int f T1 = 3 f T3 = 4 Here, does not have any missing cases, despite omittting T2, because T2 :: T Bool. This patch teaches the overlap checker about GADTs, which happily turned out to be rather easy even though the overlap checker needs a serious rewrite.
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