Add support for typer coercion for invariant type arguments such as a…

…rray payloads. (#6518)

If type `t<'a>` is invariant in `'a` allow type coercion from `t<t1> ` to `t<t2>` if both `t1` can be coerced to `t2` and `t2` can be coerced to `t1`, which must imply that the runtime representations are the same.

jscomp/ml/ctype.ml

@@ -3939,8 +3939,14 @@ let rec subtype_rec env trace t1 t2 cstrs =
               let (co, cn) = Variance.get_upper v in
               if co then
                 if cn then
-                  (trace, newty2 t1.level (Ttuple[t1]),
-                   newty2 t2.level (Ttuple[t2]), !univar_pairs) :: cstrs
+                  (* Invariant type argument: check both ways *)
+                  if
+                    subtype_rec env ((t1, t2)::trace) t1 t2 [] = [] &&
+                    subtype_rec env ((t2, t1)::trace) t2 t1 [] = [] then
+                    cstrs
+                  else
+                      (trace, newty2 t1.level (Ttuple[t1]),
+                      newty2 t2.level (Ttuple[t2]), !univar_pairs) :: cstrs
                 else subtype_rec env ((t1, t2)::trace) t1 t2 cstrs
               else
                 if cn then subtype_rec env ((t2, t1)::trace) t2 t1 cstrs

jscomp/test/Coercion.res

@@ -1,3 +1,11 @@
 let x = 1
 
 let xx = (x :> float)
+
+type r1 = {x:int}
+type r2 = {x:int}
+
+type t1 = array<r1>
+type t2 = array<r2>
+
+let foo = (x: t1) => { x :> t2 }