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<h3 style="margin-top: 20px; margin-bottom: 10px;">
Ryan Scott pushed to branch wip/T22141 at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong style="font-weight: bold;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/ed5215cd4cd2d3263e6c307c23d7d7dd9c2db6dc">ed5215cd</a></strong>
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<span> by Ryan Scott </span> <i> at 2023-10-13T17:39:59-04:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 14px; color: #333238; position: relative; font-family: var(--default-mono-font, "GitLab Mono"),"JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dcdcde;'>More robust checking for DataKinds
As observed in #22141, GHC was not doing its due diligence in catching code
that should require `DataKinds` in order to use. Most notably, it was allowing
the use of arbitrary data types in kind contexts without `DataKinds`, e.g.,
```hs
data Vector :: Nat -> Type -> Type where
```
This patch revamps how GHC tracks `DataKinds`. The full specification is
written out in the `DataKinds` section of the GHC User's Guide, and the
implementation thereof is described in `Note [Checking for DataKinds]` in
`GHC.Tc.Validity`. In brief:
* We catch _type_-level `DataKinds` violations in the renamer. See
`checkDataKinds` in `GHC.Rename.HsType` and `check_data_kinds` in
`GHC.Rename.Pat`.
* We catch _kind_-level `DataKinds` violations in the typechecker, as this
allows us to catch things that appear beneath type synonyms. (We do *not*
want to do this in type-level contexts, as it is perfectly fine for a type
synonym to mention something that requires DataKinds while still using the
type synonym in a module that doesn't enable DataKinds.) See `checkValidType`
in `GHC.Tc.Validity`.
* There is now a single `TcRnDataKindsError` that classifies all manner of
`DataKinds` violations, both in the renamer and the typechecker. The
`NoDataKindsDC` error has been removed, as it has been subsumed by
`TcRnDataKindsError`.
* I have added `CONSTRAINT` is `isKindTyCon`, which is what checks for illicit
uses of data types at the kind level without `DataKinds`. Previously,
`isKindTyCon` checked for `Constraint` but not `CONSTRAINT`. This is
inconsistent, given that both `Type` and `TYPE` were checked by `isKindTyCon`.
Moreover, it thwarted the implementation of the `DataKinds` check in
`checkValidType`, since we would expand `Constraint` (which was OK without
`DataKinds`) to `CONSTRAINT` (which was _not_ OK without `DataKinds`) and
reject it. Now both are allowed.
* I have added a flurry of additional test cases that test various corners of
`DataKinds` checking.
Fixes #22141.
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The diff was not included because it is too large.
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