Add a test for #14815:

Because the program doesn't have any binders that -XStrict can make
strict, the desugarer output should be identical when it's compiled with
and without -XStrict. This wasn't the case with GHC 8.2.2, but
apparently it was fixed some time between 8.2.2 and 8.4.1. We now add a
test case to make sure it stays fixed.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #14815

Differential Revision: https://phabricator.haskell.org/D4531

4 files changed, 57 insertions, 2 deletions

diff --git a/testsuite/tests/deSugar/should_compile/Makefile b/testsuite/tests/deSugar/should_compile/Makefile
index 792d4e7bc9..4600070c05 100644
--- a/testsuite/tests/deSugar/should_compile/Makefile
+++ b/testsuite/tests/deSugar/should_compile/Makefile
@@ -5,12 +5,21 @@ include $(TOP)/mk/test.mk
 T5252:
 	$(RM) -f T5252.hi  T5252.o
 	$(RM) -f T5252a.hi T5252a.o
-	'$(TEST_HC)' $(TEST_HC_OPTS) -c T5252a.hs 
+	'$(TEST_HC)' $(TEST_HC_OPTS) -c T5252a.hs
 	'$(TEST_HC)' $(TEST_HC_OPTS) -c T5252.hs
 
 # Failed when compiled *without* optimisation
 T5252Take2:
 	$(RM) -f T5252Take2.hi  T5252Take2.o
 	$(RM) -f T5252Take2a.hi T5252Take2a.o
-	'$(TEST_HC)' $(TEST_HC_OPTS) -c T5252Take2a.hs 
+	'$(TEST_HC)' $(TEST_HC_OPTS) -c T5252Take2a.hs
 	'$(TEST_HC)' $(TEST_HC_OPTS) -c T5252Take2.hs
+
+T14815:
+	'$(TEST_HC)' $(TEST_HC_OPTS) T14815.hs -ddump-ds -dsuppress-uniques -ddump-to-file -dumpdir lazy -fforce-recomp
+	'$(TEST_HC)' $(TEST_HC_OPTS) T14815.hs -XStrict -ddump-ds -dsuppress-uniques -ddump-to-file -dumpdir strict -fforce-recomp
+	# Drop time stamps from both files
+	tail -n +5 lazy/T14815.dump-ds >lazy_out
+	tail -n +5 strict/T14815.dump-ds >strict_out
+	# Finally compare outputs
+	diff lazy_out strict_out -q
diff --git a/testsuite/tests/deSugar/should_compile/T14815.hs b/testsuite/tests/deSugar/should_compile/T14815.hs
new file mode 100644
index 0000000000..fc5a6ee26e
--- /dev/null
+++ b/testsuite/tests/deSugar/should_compile/T14815.hs
@@ -0,0 +1,43 @@
+-- Desugarer outputs of this program when compiled with and without -XStrict
+-- should be the same because this program has only one binder (`a` in function
+-- `primitive`), but the binder is annotated with a laziness annotation, so
+-- -XStrict should have no effect on that binder.
+--
+-- Derived methods are also effected by -XStrict, but in our case we derive via
+-- GND which just generates coercions like
+--
+--     instance Functor m => Functor (StateT s m) where
+--       fmap
+--         = coerce
+--             @(forall (a_aJ2 :: TYPE LiftedRep) (b_aJ3 :: TYPE LiftedRep).
+--               a_aJ2 -> b_aJ3
+--               -> StateT s_aDW m_aDX a_aJ2 -> StateT s_aDW m_aDX b_aJ3)
+--             @(forall (a_aJ2 :: TYPE LiftedRep) (b_aJ3 :: TYPE LiftedRep).
+--               a_aJ2 -> b_aJ3
+--               -> StateT s_aDW m_aDX a_aJ2 -> StateT s_aDW m_aDX b_aJ3)
+--             fmap
+--
+-- So really -XStrict shouldn't have any effect on this program.
+
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE MagicHash                  #-}
+{-# LANGUAGE MultiParamTypeClasses      #-}
+{-# LANGUAGE TypeFamilies               #-}
+{-# LANGUAGE UnboxedTuples              #-}
+
+module K where
+
+import qualified Control.Monad.State.Strict as S
+import Control.Monad.Trans
+import GHC.Exts
+
+class Monad m => PrimMonad m where
+  type PrimState m
+  primitive :: (State# (PrimState m) -> (# State# (PrimState m), a #)) -> m a
+
+newtype StateT s m a = StateT (S.StateT s m a)
+  deriving (Functor, Applicative, Monad, MonadTrans)
+
+instance PrimMonad m => PrimMonad (StateT s m) where
+  type PrimState (StateT s m) = PrimState m
+  primitive ~a = lift (primitive a) ; {-# INLINE primitive #-}
diff --git a/testsuite/tests/deSugar/should_compile/T14815.stdout b/testsuite/tests/deSugar/should_compile/T14815.stdout
new file mode 100644
index 0000000000..f51afc4f54
--- /dev/null
+++ b/testsuite/tests/deSugar/should_compile/T14815.stdout
@@ -0,0 +1,2 @@
+[1 of 1] Compiling K                ( T14815.hs, T14815.o )
+[1 of 1] Compiling K                ( T14815.hs, T14815.o )
diff --git a/testsuite/tests/deSugar/should_compile/all.T b/testsuite/tests/deSugar/should_compile/all.T
index 2608b7d245..2d361464a6 100644
--- a/testsuite/tests/deSugar/should_compile/all.T
+++ b/testsuite/tests/deSugar/should_compile/all.T
@@ -102,3 +102,4 @@ test('T13870', normal, compile, [''])
 test('T14135', normal, compile, [''])
 test('T14773a', normal, compile, ['-Wincomplete-patterns'])
 test('T14773b', normal, compile, ['-Wincomplete-patterns'])
+test('T14815', [], run_command, ['$MAKE -s --no-print-directory T14815'])