Add test case for #8917

testsuite/tests/ghci/scripts/T8917.hs

+{-# LANGUAGE DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}
+
+module T8917 where
+
+data Nat = Zero | Succ Nat
+type family a + b where
+  Zero + a = a
+  (Succ n) + m = Succ (n + m)

testsuite/tests/ghci/scripts/T8917.script

+:load T8917
+:seti -XDataKinds -XTypeOperators
+:kind! Zero + Succ Zero
+:kind! Succ (Zero + Zero)
\ No newline at end of file

testsuite/tests/ghci/scripts/T8917.stdout

+Zero + Succ Zero :: Nat
+= 'Succ 'Zero
+Succ (Zero + Zero) :: Nat
+= 'Succ 'Zero

testsuite/tests/ghci/scripts/all.T

@@ -169,3 +169,4 @@ test('T8696', normal, ghci_script, ['T8696.script'])
 test('T8776', normal, ghci_script, ['T8776.script'])
 test('ghci059', normal, ghci_script, ['ghci059.script'])
 test('T8831', expect_broken(8831), ghci_script, ['T8831.script'])
+test('T8917', normal, ghci_script, ['T8917.script'])