testsuite: Add testsuite for #16978

(cherry picked from commit 3b31a94df4cf7e55d93dfcad9b96d0f49f4d3f8d)

2 files changed, 30 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/T16978.hs b/testsuite/tests/simplCore/should_compile/T16978.hs
new file mode 100644
index 0000000000..cf78013dfd
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T16978.hs
@@ -0,0 +1,29 @@
+-- | Caused Core Lint failure due to floating of unlifted join point to the
+-- top-level.
+module Bug where
+
+import Control.Monad.Trans.State.Strict (State, modify')
+import Data.Text (Text)
+import qualified Data.Text.Lazy.Builder as B
+
+data Value = String !Text | Null
+
+type Render a = State B.Builder a
+
+tellBuilder :: B.Builder -> Render ()
+tellBuilder b' = modify' f where
+    f b = b <> b'
+
+renderNode :: Value -> Render ()
+renderNode v =
+  renderValue v >>= outputRaw
+
+outputRaw :: Text -> Render ()
+outputRaw = tellBuilder . B.fromText
+{-# INLINE outputRaw #-}
+
+renderValue :: Value -> Render Text
+renderValue v = case v of
+    String str -> return str
+    _ -> let x = x in x
+{-# INLINE renderValue #-}
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 3302daff0c..8d6799cbad 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -327,5 +327,6 @@ test('T15631',
      run_command,
      ['$MAKE -s --no-print-directory T15631'])
 test('T15673', normal, compile, ['-O'])
+test('T16978', normal, compile, ['-O'])
 test('T16979a', normal, compile, ['-O'])
 test('T16979b', normal, compile, ['-O'])