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

(cherry picked from commit b8a797ecc34a309bd78f5a290e3554642a3a478a)

3 files changed, 15 insertions, 0 deletions

diff --git a/testsuite/tests/th/T15815A.hs b/testsuite/tests/th/T15815A.hs
new file mode 100644
index 0000000000..4025f3856c
--- /dev/null
+++ b/testsuite/tests/th/T15815A.hs
@@ -0,0 +1,7 @@
+{-# LANGUAGE TemplateHaskell #-}
+module T15815A where
+
+mkFoo tyQ = [d|
+    foo :: a ~ $(tyQ) => a
+    foo = undefined
+  |]
diff --git a/testsuite/tests/th/T15815B.hs b/testsuite/tests/th/T15815B.hs
new file mode 100644
index 0000000000..32bf2edfd5
--- /dev/null
+++ b/testsuite/tests/th/T15815B.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TemplateHaskell, GADTs #-}
+module T15815B where
+
+import T15815A
+
+mkFoo [t| Int -> Int |]
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 6e564461e0..ddc2708f1e 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -425,3 +425,5 @@ test('T15550', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('T15572', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('T15481', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('TH_recover_warns', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
+test('T15815', normal, multimod_compile,
+    ['T15815B', '-v0 ' + config.ghc_th_way_flags])