DeriveGeneric: look up datacon fixities using getDataConFixityFun

Previously, `DeriveGeneric` would look up the fixity of a data constructor
using `getFixityEnv`, but this is subtly incorrect for data constructors
defined in external modules. This sort of situation can happen with
`StandaloneDeriving`, as noticed in #20994. In fact, the same bug has occurred
in the past in #9830, and while that bug was fixed for `deriving Read` and
`deriving Show`, the fix was never extended to `DeriveGeneric` due to an
oversight. This patch corrects that oversight.

Fixes #20994.

2 files changed, 28 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T20994.hs b/testsuite/tests/deriving/should_compile/T20994.hs
new file mode 100644
index 0000000000..807f16a9b6
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T20994.hs
@@ -0,0 +1,27 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+module T20994 where
+
+import Data.Kind (Constraint, Type)
+import GHC.Generics
+
+type (++) :: [a] -> [a] -> [a]
+type family xs ++ ys where
+  '[]    ++ ys = ys
+  (x:xs) ++ ys = x:(xs ++ ys)
+
+type GetConFixities :: (Type -> Type) -> [FixityI]
+type family GetConFixities rep where
+  GetConFixities (D1 (MetaData _ _ _ _) f) = GetConFixities f
+  GetConFixities (C1 (MetaCons _ fix _) _) = '[fix]
+  GetConFixities (f :+: g)                 = GetConFixities f ++ GetConFixities g
+  GetConFixities V1                        = '[]
+
+type Dict :: Constraint -> Type
+data Dict c where
+  Dict :: c => Dict c
+
+-- Check that (:) is `infixr 5` according to its Rep instance.
+test :: Dict (GetConFixities (Rep [a]) ~ [PrefixI, InfixI RightAssociative 5])
+test = Dict
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 4a80c84cc1..a33cb364c3 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -136,3 +136,4 @@ test('T20496', multiline_grep_errmsg(r"rnd\n( .*\n)*"), compile, ['-ddump-tc-tra
 test('T20375', normal, compile, [''])
 test('T20387', normal, compile, [''])
 test('T20501', normal, compile, [''])
+test('T20994', normal, compile, [''])