Conservatively estimate levity in worker/wrapper

The worker/wrapper transform needs to determine the levity of the result to
determine whether it needs to introduce a lambda to preserve laziness of the
result. For this is previously used isUnliftedType. However, this may fail in
the presence of levity polymorphism.

We now instead use isLiftedType_maybe, assuming that a lambda is needed if the
levity of the result cannot be determined.

Fixes #15186.

Test Plan: make test=T15186

Reviewers: simonpj, goldfire, tdammers

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15186

Differential Revision: https://phabricator.haskell.org/D4755

compiler/stranal/WwLib.hs

@@ -269,11 +269,21 @@ mkWorkerArgs dflags args res_ty
     | otherwise
     = (args ++ [voidArgId], args ++ [voidPrimId])
     where
+      -- See "Making wrapper args" section above
       needsAValueLambda =
-        isUnliftedType res_ty
+        lifted
+        -- We may encounter a levity-polymorphic result, in which case we
+        -- conservatively assume that we have laziness that needs preservation.
+        -- See #15186.
         || not (gopt Opt_FunToThunk dflags)
            -- see Note [Protecting the last value argument]
 
+      -- Might the result be lifted?
+      lifted =
+        case isLiftedType_maybe res_ty of
+          Just lifted -> lifted
+          Nothing     -> True
+
 {-
 Note [Protecting the last value argument]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

testsuite/tests/simplCore/should_compile/all.T

@@ -315,4 +315,4 @@ test('T15002', [ req_profiling ], compile, ['-O -fprof-auto -prof'])
 test('T15005', normal, compile, ['-O'])
 # we omit profiling because it affects the optimiser and makes the test fail
 test('T15056', [extra_files(['T15056a.hs']), omit_ways(['profasm'])], multimod_compile, ['T15056', '-O -v0 -ddump-rule-firings'])
-test('T15186', expect_broken(15186), multimod_compile, ['T15186', '-v0'])
+test('T15186', normal, multimod_compile, ['T15186', '-v0'])