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Summary: Improve the way `(!)`, `(~)`, and other type operators are handled in the parser, fixing two issues at once: 1. `(!)` can now be used as a type operator that respects fixity and precedence (#15457) 2. Existential context of a data constructor no longer needs parentheses (#15675) In addition to that, with this patch it is now trivial to adjust precedence of the `{-# UNPACK #-}` pragma, as suggested in https://ghc.haskell.org/trac/ghc/ticket/14761#comment:7 There was a small change to API Annotations. Before this patch, `(~)` was a strange special case that produced an annotation unlike any other type operator. After this patch, when `(~)` or `(!)` are used to specify strictness they produce AnnTilde and AnnBang annotations respectively, and when they are used as type operators, they produce no annotations. Test Plan: Validate Reviewers: simonpj, bgamari, alanz, RyanGlScott Reviewed By: RyanGlScott Subscribers: RyanGlScott, rwbarton, mpickering, carter GHC Trac Issues: #15457, #15675 Differential Revision: https://phabricator.haskell.org/D5180
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