The command has indeed failed with message:
In environment
list' : Set -> Set
A : Set
a : A
l : list' A
Unable to unify "list' (A * A)%type" with "list' A".
Inductive foo (A : Type) (x : A) (y : A := x) : Prop :=  Foo : foo A x.

Arguments foo _%type_scope _
Arguments Foo _%type_scope _
myprod unit bool
     : Set