Fix #16666 by parenthesizing contexts in Convert

Most places where we convert contexts in `Convert` are actually in
positions that are to the left of some `=>`, such as in superclasses
and instance contexts. Accordingly, these contexts need to be
parenthesized at `funPrec`. To accomplish this, this patch changes
`cvtContext` to require a precedence argument for the purposes of
calling `parenthesizeHsContext` and adjusts all `cvtContext` call
sites accordingly.

3 files changed, 19 insertions, 0 deletions

diff --git a/testsuite/tests/th/T16666.hs b/testsuite/tests/th/T16666.hs
new file mode 100644
index 0000000000..88351fd5a7
--- /dev/null
+++ b/testsuite/tests/th/T16666.hs
@@ -0,0 +1,11 @@
+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE UndecidableInstances #-}
+module T16666 where
+
+$([d| class    (c => d) => Implies c d
+      instance (c => d) => Implies c d
+    |])
diff --git a/testsuite/tests/th/T16666.stderr b/testsuite/tests/th/T16666.stderr
new file mode 100644
index 0000000000..8264967396
--- /dev/null
+++ b/testsuite/tests/th/T16666.stderr
@@ -0,0 +1,7 @@
+T16666.hs:(9,3)-(11,6): Splicing declarations
+    [d| class (c => d) => Implies c d
+        
+        instance (c => d) => Implies c d |]
+  ======>
+    class (c => d) => Implies c d
+    instance (c => d) => Implies c d
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index f9738615c7..37d21c3707 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -473,3 +473,4 @@ test('T16195', normal, multimod_compile, ['T16195.hs', '-v0'])
 test('T16293b', normal, compile, [''])
 test('T16326_TH', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('T14741', normal, compile_and_run, [''])
+test('T16666', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])