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Ryan Scott authored
An unfortunate consequence of commit b9483981 (`Remove HsEqTy and XEqTy`) is infix uses of `~` in TH quotes now desugar differently than before. In particular, we have that: ```haskell a ~ (Int -> Int) ``` Now desugars to: ```haskell HsOpTy a (~) (HsOpTy Int (->) Int) ``` Which GHC interprets as being: ```haskell a ~ Int -> Int ``` Or, equivalently: ```haskell (a ~ Int) -> Int ``` Which is different than what was intended! This is the cause of #15815. All of this has revealed that we likely need to renovate the way we desugar infix type operators to be more consistent with the treatment for infix expressions (see https://ghc.haskell.org/trac/ghc/ticket/15815#comment:5 for more on this.) Doing so would constitute a breaking change, however, so we will likely want to wait until another major GHC release to do this. In the meantime, this patch offers a non-invasive change to the way that infix uses of `~` are desugared. This makes the program in #15815 compile again by inserting extra `HsParTy`s around the arguments to `~` if they are lacking them. Test Plan: make test TEST=T15815 Reviewers: int-index, goldfire, bgamari Reviewed By: int-index Subscribers: int-e, rwbarton, carter GHC Trac Issues: #15815 Differential Revision: https://phabricator.haskell.org/D5274
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