Regression test for Trac #10390

testsuite/tests/typecheck/should_compile/T10390.hs

+{-# LANGUAGE RankNTypes #-}
+
+module T10390 where
+
+class ApPair r where
+  apPair :: (forall a . (ApPair a, Num a) => Maybe a) -> Maybe r
+
+instance (ApPair a, ApPair b) => ApPair (a,b) where
+  apPair = apPair'
+
+apPair' :: (ApPair b, ApPair c)
+        => (forall a . (Num a, ApPair a) => Maybe a) -> Maybe (b,c)
+            -- NB constraints in a different order to apPair
+apPair' f =  let (Just a) = apPair f
+                 (Just b) = apPair f
+          in Just $ (a, b)

testsuite/tests/typecheck/should_compile/all.T

@@ -446,3 +446,4 @@ test('T10109', normal, compile, [''])
 test('T10335', normal, compile, [''])
 test('T10489', normal, compile, [''])
 test('T10564', normal, compile, [''])
+test('T10390', normal, compile, [''])