Fix variance composition (#12623)

Possible positive occurrence under possible negative occurrence were
forgotten, making the typechecker accepts the absurd statement

    type -'a n
    type +'a p
    type +'a ko = 'a p n

(cherry picked from commit 1a9c453)

Author: Octachron

Changes

@@ -1,3 +1,11 @@
+OCaml 5.1.1
+-----------
+
+### Bug fixes:
+
+- #12623, fix the computation of variance composition
+  (Florian Angeletti, report by Vesa Karvonen, review by Gabriel Scherer)
+
 OCaml 5.1.0 (14 September 2023)
 -------------------------------
 

boot/ocamlc

@@ -36,3 +36,90 @@ type !'a t = (module s with type t = 'a);;
 module type s = sig type t end
 type 'a t = (module s with type t = 'a)
 |}]
+
+(* Composition *)
+type -'a n
+type +'a p
+type !'a i
+
+type +'a error_np = 'a n p;;
+[%%expect{|
+type -'a n
+type +'a p
+type !'a i
+Line 5, characters 0-26:
+5 | type +'a error_np = 'a n p;;
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be covariant,
+       but it is contravariant.
+|}]
+
+
+type +'a error_pn = 'a p n;;
+[%%expect{|
+Line 1, characters 0-26:
+1 | type +'a error_pn = 'a p n;;
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be covariant,
+       but it is contravariant.
+|}]
+
+type -'a error_pp = 'a p p;;
+[%%expect{|
+Line 1, characters 0-26:
+1 | type -'a error_pp = 'a p p;;
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be contravariant,
+       but it is covariant.
+|}]
+
+type -'a error_nn = 'a n n;;
+[%%expect{|
+Line 1, characters 0-26:
+1 | type -'a error_nn = 'a n n;;
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be contravariant,
+       but it is covariant.
+|}]
+
+type !'a inj_in = 'a i n
+[%%expect{|
+Line 1, characters 0-24:
+1 | type !'a inj_in = 'a i n
+    ^^^^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be injective invariant,
+       but it is invariant.
+|}]
+
+type !'a inj_in = 'a n i
+[%%expect{|
+Line 1, characters 0-24:
+1 | type !'a inj_in = 'a n i
+    ^^^^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be injective invariant,
+       but it is invariant.
+|}]
+
+module Make_covariant(M: sig type 'a t end): sig
+  type 'a i = 'a
+  type +'a t = 'a i M.t
+end = struct
+  type 'a i = 'a
+  type +'a t = 'a i M.t
+end
+
+module Positive_ref = Make_covariant(struct type 'a t = 'a ref end)
+[%%expect {|
+Line 6, characters 2-23:
+6 |   type +'a t = 'a i M.t
+      ^^^^^^^^^^^^^^^^^^^^^
+Error: In this definition, expected parameter variances are not satisfied.
+       The 1st type parameter was expected to be covariant,
+       but it is invariant.
+|}]

boot/ocamllex

@@ -186,7 +186,7 @@ module Variance = struct
     let mp =
       mem May_pos v1 && mem May_pos v2 || mem May_neg v1 && mem May_neg v2
     and mn =
-      mem May_pos v1 && mem May_neg v2 || mem May_pos v1 && mem May_neg v2
+      mem May_pos v1 && mem May_neg v2 || mem May_neg v1 && mem May_pos v2
     and mw = mem May_weak v1 && v2 <> null || v1 <> null && mem May_weak v2
     and inj = mem Inj v1 && mem Inj v2
     and pos = mem Pos v1 && mem Pos v2 || mem Neg v1 && mem Neg v2