Add test for #21558

This is now fixed on master and 9.2 branch.

Closes #21558

(cherry picked from commit 0e2d16eb76037152c96226f0f65a5ebdee64f7b6)

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 57110891f1..ae0a917629 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -788,6 +788,7 @@ test('T18467', normal, compile, [''])
 test('T19315', normal, compile, [''])
 
 test('T19535', normal, compile, [''])
+test('T21558', normal, compile, ['-w'])
 test('T21516', normal, compile, [''])
 test('T21519', normal, compile, [''])
 test('T21519a', normal, compile, [''])