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GHC had an ad hoc validity check in place to rule out pattern variables bound by type synonyms, such as in the following example: ```hs type ItemColID a b = Int -- Discards a,b get :: ItemColID a b -> ItemColID a b get (x :: ItemColID a b) = x :: ItemColID a b ``` This hack is wholly unnecessary nowadays, since OutsideIn(X) is more than capable of instantiating `a` and `b` to `Any`. In light of this, let's rip out this validity check. Fixes #17007.
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