  Sound exhaustiveness checking of pattern-matching is an essential feature of
functional programming languages, and OCaml supports it for GADTs. However this
check is incomplete, in that it may fail to detect that a pattern can match no
concrete value. In this paper we show that this problem is actually
undecidable, but that we can strengthen the exhaustiveness and redundancy
checks so that they cover more practical cases. The new algorithm relies on a
clever modification of type inference for patterns.
