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Adapt to DLamCasesE in th-desugar-1.18
This patch bumps the pinned `th-desugar` commit to bring in the changes from goldfirere/th-desugar#218, which replaces `DLamE` and `DCaseE` in favor of `DLamCasesE`. Quite happily, this actually _simplifies_ how singling works in `singletons-th`. Previously, we went through great efforts to annotate promoted `case` expressions with their overall return type, as described in `Note [Annotate case return type]` and `Note [The id hack; or, how singletons-th learned to stop worrying and avoid kind generalization]` in `D.S.TH.Single`. After this patch, however, all `case` expressions are desugared to `\cases` expressions, and because we already annotate singled `\cases` expressions (by generating code like `singFun1 @LamCasesSym0 (\cases ...)`), we no longer need to use the tricks describes in those two Notes. Hooray! One interesting knock-on effect of these simplifications is that given code like this: ```hs map (\x -> ()) xs -- Note that `x` is unused ``` `singletons-th` would previously generate code that looked like this: ```hs sMap (singFun1 @LamdaSym0 (\sX -> case sX of (_ :: Sing x) -> STuple0)) sXs ``` Unlike the original code, this singled code would compile without warnings. This is because `sX` is "used" in the sense that it is passed to a `case` expression. It's a very silly `case` expression, as it doesn't do anything meaningful with `sX`, but it still technically counts as a use. Now that `singletons-th` uses a `\cases`-based approach to singling, however, the singled code will instead look like this: ```hs sMap (singFun1 @LamCasesSym0 (\cases (sX :: Sing x) -> STuple0)) sXs ``` This time, GHC _does_ recognize that `sX` is unused and emits a warning. This actually affects the way that `singletons-th` generates code for derived `Functor` and `Foldable` instances, so I needed to generate wildcard patterns instead of variable patterns in certain parts of `D.S.TH.Promote.Deriving.{Functor,Foldable}`. I have also mentioned the possibility of `singletons-th` generating more warnings in the `CHANGELOG`.
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singletons-base/tests/compile-and-dump/GradingClient/Database.golden
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