Commit d584e3f0 authored by Committed by Marge Bot
Use addUsedDataCons more judiciously in TcDeriv (#17324)
If you derive an instance like this: ```hs deriving <...> instance Foo C ``` And the data constructors for `C` aren't in scope, then `doDerivInstErrorChecks1` throws an error. Moreover, it will _only_ throw an error if `<...>` is either `stock` or `newtype`. This is because the code that the `anyclass` or `via` strategies would generate would not require the use of the data constructors for `C`. However, `doDerivInstErrorChecks1` has another purpose. If you write this: ```hs import M (C(MkC1, ..., MkCn)) deriving <...> instance Foo C ``` Then `doDerivInstErrorChecks1` will call `addUsedDataCons` on `MkC1` through `MkCn` to ensure that `-Wunused-imports` does not complain about them. However, `doDerivInstErrorChecks1` was doing this for _every_ deriving strategy, which mean that if `<...>` were `anyclass` or `via`, then the warning about `MkC1` through `MkCn` being unused would be suppressed! The fix is simple enough: only call `addUsedDataCons` when the strategy is `stock` or `newtype`, just like the other code paths in `doDerivInstErrorChecks1`. Fixes #17324.
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