Test Trac #9708

testsuite/tests/typecheck/should_compile/T9708.hs

+{-# LANGUAGE DataKinds, TypeOperators, TypeFamilies #-}
+module TcTypeNatSimple where
+
+import GHC.TypeLits
+import Data.Proxy
+
+type family SomeFun (n :: Nat)
+
+ti7 :: (x <= y, y <= x) => Proxy (SomeFun x) -> Proxy y -> ()
+ti7 _ _ = ()

testsuite/tests/typecheck/should_compile/T9708.stderr

+
+T9708.hs:9:8:
+    Could not deduce (SomeFun x ~ SomeFun y)
+    from the context (x <= y, y <= x)
+      bound by the type signature for
+                 ti7 :: (x <= y, y <= x) => Proxy (SomeFun x) -> Proxy y -> ()
+      at T9708.hs:9:8-61
+    NB: ‘SomeFun’ is a type function, and may not be injective
+    Expected type: Proxy (SomeFun x) -> Proxy y -> ()
+      Actual type: Proxy (SomeFun y) -> Proxy y -> ()
+    In the ambiguity check for:
+      forall (x :: Nat) (y :: Nat).
+      (x <= y, y <= x) =>
+      Proxy (SomeFun x) -> Proxy y -> ()
+    To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
+    In the type signature for ‘ti7’:
+      ti7 :: (x <= y, y <= x) => Proxy (SomeFun x) -> Proxy y -> ()

testsuite/tests/typecheck/should_compile/all.T

@@ -421,3 +421,4 @@ test('MutRec', normal, compile, [''])
 test('T8856', normal, compile, [''])
 test('T9117', normal, compile, [''])
 test('T9117_2', expect_broken('9117'), compile, [''])
+test('T9708', normal, compile_fail, [''])