Add a minimized regression test for #12928

testsuite/tests/typecheck/should_compile/T12928.hs

+{-# LANGUAGE DataKinds, PolyKinds #-}
+
+module T12928 where
+
+data P (a::k) = MkP
+
+data FffSym0 (l :: P a)
+
+-- Make sure that the kind of 'k' is not defaulted:
+--
+--    data FffSym0 (l :: P (a :: Type))
+--
+-- We expect kind polymorphism:
+--
+--    data FffSym0 (l :: P (a :: k))
+--
+type Inst (a :: P Either) (b :: P Maybe) = (FffSym0 a, FffSym0 b)

testsuite/tests/typecheck/should_compile/all.T

@@ -673,3 +673,4 @@ test('T13951', normal, compile, [''])
 test('T16411', normal, compile, [''])
 test('T16609', normal, compile, [''])
 test('T505', normal, compile, [''])
+test('T12928', normal, compile, [''])