Add test for #21871

This adds a test for #21871, which was fixed by the No Skolem Info
rework (MR !7105).

Fixes #21871

3 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_fail/T21871.hs b/testsuite/tests/deriving/should_fail/T21871.hs
new file mode 100644
index 0000000000..83c56d696e
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21871.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE DerivingVia, StandaloneDeriving #-}
+
+module T21871 where
+
+import Control.Monad.Trans.Reader
+  ( ReaderT(ReaderT) )
+import Data.Kind
+  ( Type )
+
+newtype FooT m a = FooT
+  { runFooT :: Int -> m a
+  }
+  deriving Functor via ReaderT Int m
diff --git a/testsuite/tests/deriving/should_fail/T21871.stderr b/testsuite/tests/deriving/should_fail/T21871.stderr
new file mode 100644
index 0000000000..3ee6692bd6
--- /dev/null
+++ b/testsuite/tests/deriving/should_fail/T21871.stderr
@@ -0,0 +1,9 @@
+
+T21871.hs:13:36: error:
+    • Couldn't match kind ‘k’ with ‘*’
+      Expected kind ‘* -> *’, but ‘m’ has kind ‘k -> *’
+      ‘k’ is a rigid type variable bound by
+        the newtype declaration for ‘FooT’
+        at T21871.hs:(10,1)-(13,36)
+    • In the second argument of ‘ReaderT’, namely ‘m’
+      In the newtype declaration for ‘FooT’
diff --git a/testsuite/tests/deriving/should_fail/all.T b/testsuite/tests/deriving/should_fail/all.T
index 2b2f2af562..656e4dd5c2 100644
--- a/testsuite/tests/deriving/should_fail/all.T
+++ b/testsuite/tests/deriving/should_fail/all.T
@@ -83,3 +83,4 @@ test('deriving-via-fail5', normal, compile_fail, [''])
 test('T21087', normal, compile_fail, [''])
 test('T21087b', [extra_files(['T21087b_aux.hs','T21087b_aux.hs-boot'])], multimod_compile_fail, ['T21087b', ''])
 test('T21302', normal, compile_fail, [''])
+test('T21871', normal, compile_fail, [''])