Primitive projections break guarantee that [Set Implicit Arguments] only makes the arguments that can truly be inferred implicit (polyproj)

[JasonGross]

Set Primitive Projections.
Set Implicit Arguments.
Set Universe Polymorphism.

Inductive sum A B := inl : A -> sum A B | inr : B -> sum A B.
Inductive Empty :=.

Record category :=
  { ob :> Type;
    hom : ob -> ob -> Type
  }.

Definition sum_category (C D : category) : category :=
  {|
    ob := sum (ob C) (ob D);
    hom x y := match x, y with
                 | inl x, inl y => hom x y
                 | inr x, inr y => hom x y
                 | _, _ => Empty
               end |}.

Goal forall C D (x y : ob (sum_category C D)), Type.
intros C D x y.
hnf in x, y.
exact (hom x y). (* Toplevel input, characters 26-27:
Error:
In environment
C : category
D : category
x : sum (ob C) (ob D)
y : sum (ob C) (ob D)
The term "x" has type "sum (ob C) (ob D)" while it is expected to have type
 "ob ?16". *)

Also, it's backwards-incompatible behavior for Set Implicit Arguments.

[mattam82]

Fixed in trunk (upcoming commit)