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Diffstat (limited to 'compiler/GHC/Rename/Module.hs')
| -rw-r--r-- | compiler/GHC/Rename/Module.hs | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/compiler/GHC/Rename/Module.hs b/compiler/GHC/Rename/Module.hs index ff4f3cce8e..bc14c1b189 100644 --- a/compiler/GHC/Rename/Module.hs +++ b/compiler/GHC/Rename/Module.hs @@ -2154,6 +2154,28 @@ The main parts of the implementation are: If `T` were an ordinary `data` declaration, then `A` would have a wrapper to account for the GADT-like equality in its return type. Because `T` is declared as a `type data` declaration, however, the wrapper is omitted. + +* Although `type data` data constructors do not exist at the value level, + it is still possible to match on a value whose type is headed by a `type data` + type constructor, such as this example from #22964: + + type data T a where + A :: T Int + B :: T a + + f :: T a -> () + f x = case x of {} + + This has two consequences: + + * During checking the coverage of `f`'s pattern matches, we treat `T` as if it + were an empty data type so that GHC does not warn the user to match against + `A` or `B`. (Otherwise, you end up with the bug reported in #22964.) + See GHC.HsToCore.Pmc.Solver.vanillaCompleteMatchTC. + + * In `GHC.Core.Utils.refineDataAlt`, do /not/ fill in the DEFAULT case with + the data constructor. See + Note [Refine DEFAULT case alternatives] Exception 2, in GHC.Core.Utils. -} warnNoDerivStrat :: Maybe (LDerivStrategy GhcRn) |