RnExpr: Fix ApplicativeDo desugaring with RebindableSyntax

We need to compare against the local return and pure, not returnMName
and pureAName.

Fixes #12490.

Test Plan: Validate, add testcase

Reviewers: austin, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2499

GHC Trac Issues: #12490

2 files changed, 31 insertions, 0 deletions

diff --git a/testsuite/tests/ado/T12490.hs b/testsuite/tests/ado/T12490.hs
new file mode 100644
index 0000000000..e1bb022990
--- /dev/null
+++ b/testsuite/tests/ado/T12490.hs
@@ -0,0 +1,30 @@
+{-# LANGUAGE RebindableSyntax #-}
+{-# LANGUAGE ApplicativeDo #-}
+
+module T12490 where
+
+import Prelude (Int, String, Functor(..), ($), undefined, (+))
+
+join :: Monad f => f (f a) -> f a
+join = undefined
+
+class Functor f => Applicative f where
+    pure :: a -> f a
+    (<*>) :: f (a -> b) -> f a -> f b
+
+class Applicative f => Monad f where
+    return :: a -> f a
+    (>>=) :: f a -> (a -> f b) -> f b
+    fail :: String -> f a
+
+f_app :: Applicative f => f Int -> f Int -> f Int
+f_app a b = do
+    a' <- a
+    b' <- b
+    pure (a' + b')
+
+f_monad :: Monad f => f Int -> f Int -> f Int
+f_monad a b = do
+    a' <- a
+    b' <- b
+    return $ a' + b'
diff --git a/testsuite/tests/ado/all.T b/testsuite/tests/ado/all.T
index 06cdbf993d..67697b93e4 100644
--- a/testsuite/tests/ado/all.T
+++ b/testsuite/tests/ado/all.T
@@ -7,3 +7,4 @@ test('ado006', normal, compile, [''])
 test('ado007', normal, compile, [''])
 test('T11607', normal, compile_and_run, [''])
 test('ado-optimal', normal, compile_and_run, [''])
+test('T12490', normal, compile, [''])