Fix #15815 by parenthesizing the arguments to infix ~

An unfortunate consequence of commit
b9483981d128f55d8dae3f434f49fa6b5b30c779 (`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

1 files changed, 7 insertions, 1 deletions

diff --git a/compiler/hsSyn/Convert.hs b/compiler/hsSyn/Convert.hs
index 8b12a78ed3..06d5d6ca7a 100644
--- a/compiler/hsSyn/Convert.hs
+++ b/compiler/hsSyn/Convert.hs
@@ -1437,7 +1437,13 @@ cvtTypeKind ty_str ty
 
            EqualityT
              | [x',y'] <- tys' ->
-                   returnL (HsOpTy noExt x' (noLoc eqTyCon_RDR) y')
+                   let px = parenthesizeHsType opPrec x'
+                       py = parenthesizeHsType opPrec y'
+                   in returnL (HsOpTy noExt px (noLoc eqTyCon_RDR) py)
+               -- The long-term goal is to remove the above case entirely and
+               -- subsume it under the case for InfixT. See #15815, comment:6,
+               -- for more details.
+
              | otherwise ->
                    mk_apps (HsTyVar noExt NotPromoted
                             (noLoc eqTyCon_RDR)) tys'