Add test for #21558wip/t21355

This is now fixed on master and 9.2 branch.

Closes #21558

2 files changed, 17 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T21558.hs b/testsuite/tests/typecheck/should_compile/T21558.hs
new file mode 100644
index 0000000000..156590e640
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21558.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+module T21558 where
+
+import Data.Map
+
+type family IxValue (m :: *) :: *
+type family Index (m :: *) :: *
+
+data Op a where
+  Insert :: (a ~ Map (Index a) (IxValue a)) => (Int, Index a, IxValue a) -> Op a
+
+instance Eq (Op a) where
+
+instance (Ord (Index a), Ord (IxValue a), Ord a) => Ord (Op a) where
+  compare (Insert a1) (Insert b1) = compare a1 b1
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 4f546c0914..e9596fb6f4 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -804,6 +804,7 @@ test('T20187a', normal, compile, ['-Wredundant-strictness-flags'])
 test('T20187b', normal, compile, ['-Wredundant-strictness-flags'])
 test('T20356', normal, compile, [''])
 test('T20584', normal, compile, [''])
+test('T21558', normal, compile, ['-w'])
 test('T20584b', normal, compile, [''])
 test('T20588b', [extra_files(['T20588b.hs', 'T20588b.hs-boot', 'T20588b_aux.hs'])], multimod_compile, ['T20588b_aux.hs', '-v0'])
 test('T20588d', [extra_files(['T20588d.hs', 'T20588d.hs-boot', 'T20588d_aux.hs'])], multimod_compile, ['T20588d_aux.hs', '-v0'])