Add regression test for T20189

Closes #20189

3 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T20189.hs b/testsuite/tests/typecheck/should_fail/T20189.hs
new file mode 100644
index 0000000000..9c7e7ceae8
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20189.hs
@@ -0,0 +1,7 @@
+{-# LANGUAGE ImpredicativeTypes #-}
+{-# LANGUAGE TypeFamilies #-}
+module T20189 where
+
+y :: (t ~ (forall x . Show x => x -> IO ())) => t
+y = _
+
diff --git a/testsuite/tests/typecheck/should_fail/T20189.stderr b/testsuite/tests/typecheck/should_fail/T20189.stderr
new file mode 100644
index 0000000000..df67fa4066
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20189.stderr
@@ -0,0 +1,12 @@
+
+T20189.hs:6:5: error:
+    • Found hole: _ :: t
+      Where: ‘t’ is a rigid type variable bound by
+               the type signature for:
+                 y :: forall t. (t ~ (forall x. Show x => x -> IO ())) => t
+               at T20189.hs:5:1-49
+    • In an equation for ‘y’: y = _
+    • Relevant bindings include y :: t (bound at T20189.hs:6:1)
+      Constraints include
+        t ~ (forall x. Show x => x -> IO ()) (from T20189.hs:5:1-49)
+      Valid hole fits include y :: t (bound at T20189.hs:6:1)
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 18b07edc87..908edcfe27 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -627,3 +627,4 @@ test('OrdErr', normal, compile_fail, [''])
 test('T20542', normal, compile_fail, [''])
 test('T20588', [extra_files(['T20588.hs', 'T20588.hs-boot', 'T20588_aux.hs'])], multimod_compile_fail, ['T20588_aux.hs', '-v0'])
 test('T20588c', [extra_files(['T20588c.hs', 'T20588c.hs-boot', 'T20588c_aux.hs'])], multimod_compile_fail, ['T20588c_aux.hs', '-v0'])
+test('T20189', normal, compile_fail, [''])