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This implements chunks (2) and (3) of ghc/ghc#16762 (comment 270170). Namely, it introduces a dedicated `HsPatSigType` AST type, which represents the types that can appear in pattern signatures and term-level `RULE` binders. Previously, these were represented with `LHsSigWcType`. Although `LHsSigWcType` is isomorphic to `HsPatSigType`, the intended semantics of the two types are slightly different, as evidenced by the fact that they have different code paths in the renamer and typechecker. See also the new `Note [Pattern signature binders and scoping]` in `GHC.Hs.Types`.
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