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Ryan Scott authored
Previously, `hsScopedTvs` (and its cousin `hsWcScopedTvs`) pretended that visible dependent quantification could not possibly happen at the term level, and cemented that assumption with an `ASSERT`: ```hs hsScopedTvs (HsForAllTy { hst_fvf = vis_flag, ... }) = ASSERT( vis_flag == ForallInvis ) ... ``` It turns out that this assumption is wrong. You can end up tripping this `ASSERT` if you stick it to the man and write a type for a term that uses visible dependent quantification anyway, like in this example: ```hs {-# LANGUAGE ScopedTypeVariables #-} x :: forall a -> a -> a x = x ``` That won't typecheck, but that's not the point. Before the typechecker has a chance to reject this, the renamer will try to use `hsScopedTvs` to bring `a` into scope over the body of `x`, since `a` is quantified by a `forall`. This, in turn, causes the `ASSERT` to fail. Bummer. Instead of walking on this dangerous ground, this patch makes GHC adopt a more hardline stance by pattern-matching directly on `ForallInvis` in `hsScopedTvs`: ```hs hsScopedTvs (HsForAllTy { hst_fvf = ForallInvis, ... }) = ... ``` Now `a` will not be brought over the body of `x` at all (which is how it should be), there's no chance of the `ASSERT` failing anymore (as it's gone), and best of all, the behavior of `hsScopedTvs` does not change. Everyone wins! Fixes #17687.
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