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Previously datacon strictness was accounted for when we demand analysed a case analysis. However, this results in pessimistic demands in some cases. For instance, consider the program (from T10482) data family Bar a data instance Bar (a, b) = BarPair !(Bar a) !(Bar b) newtype instance Bar Int = Bar Int foo :: Bar ((Int, Int), Int) -> Int -> Int foo f k = case f of BarPair x y -> case burble of True -> case x of BarPair p q -> ... False -> ... We really should be able to assume that `p` is already evaluated since it came from a strict field of BarPair. However, as written the demand analyser can not conclude this since we may end up in the False branch of the case on `burble` (which places no demand on `x`). By accounting for the data con strictness later, applied to the demand of the RHS, we get the strict demand signature we want. See Note [Add demands for strict constructors] for a more comprehensive discussion. Test Plan: Validate Reviewers: simonpj, osa1, goldfire Subscribers: rwbarton, carter GHC Trac Issues: #15696 Differential Revision: https://phabricator.haskell.org/D5226
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