testsuite: Add testcase for #16394wip/T16394

3 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T16394.hs b/testsuite/tests/typecheck/should_fail/T16394.hs
new file mode 100644
index 0000000000..76ca7a7cf6
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T16394.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE PolyKinds, TypeFamilies, DataKinds #-}
+
+class C a where
+    type T (n :: a)
+
+instance C a => C b => C (a, b) where
+    type T '(n, m) = (T n, T m)
+
+-- but this worked fine:
+--
+-- instance (C a, C b) => C (a, b) where
+--   type T '(n, m) = (T n, T m)
diff --git a/testsuite/tests/typecheck/should_fail/T16394.stderr b/testsuite/tests/typecheck/should_fail/T16394.stderr
new file mode 100644
index 0000000000..fff51a6e39
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T16394.stderr
@@ -0,0 +1,5 @@
+T16394.hs:6:10: error:
+    Illegal class instance: ‘C a => C b => C (a, b)’
+      Class instances must be of the form
+        context => C ty_1 ... ty_n
+      where ‘C’ is a class
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 1a775d3a7c..b92d99ce8f 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -510,3 +510,4 @@ test('T16059e', [extra_files(['T16059b.hs'])], multimod_compile_fail,
     ['T16059e', '-v0'])
 test('T16255', normal, compile_fail, [''])
 test('T16204c', normal, compile_fail, [''])
+test('T16394', normal, compile, [''])