(* correct failure of injection/discriminate on types whose inductive status derives from the substitution of an argument *)
Unset Structural Injection.
Inductive t : nat -> Type :=
| M : forall n: nat, nat -> t n.

Lemma eq_t : forall n n' m m',
existT (fun B : Type => B) (t n) (M n m) =
existT (fun B : Type => B) (t n') (M n' m') -> True.