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The type-variables-escaping-their-scope-via-kinds check in `TcValidity` was failing to properly expand type synonyms, which led to #16391. This is easily fixed by using `occCheckExpand` before performing the validity check. Along the way, I refactored this check out into its own function, and sprinkled references to Notes to better explain all of the moving parts. Many thanks to @simonpj for the suggestions. Bumps the haddock submodule.
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