From 1b46dbe329e5b3b8ed47038a87990a4cd28e0d29 Mon Sep 17 00:00:00 2001
From: Cristiano Calcagno <cristianoc@users.noreply.github.com>
Date: Mon, 11 Dec 2023 09:55:40 +0100
Subject: [PATCH] Add support for typer coercion for invariant type arguments
 such as array payloads.

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       | 10 ++++++++--
 jscomp/test/Coercion.js  |  5 +++++
 jscomp/test/Coercion.res |  8 ++++++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/jscomp/ml/ctype.ml b/jscomp/ml/ctype.ml
index bb5154738a..7dd114fed8 100644
--- a/jscomp/ml/ctype.ml
+++ b/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
diff --git a/jscomp/test/Coercion.js b/jscomp/test/Coercion.js
index 8f99712b85..7e2ca2ca8b 100644
--- a/jscomp/test/Coercion.js
+++ b/jscomp/test/Coercion.js
@@ -2,10 +2,15 @@
 'use strict';
 
 
+function foo(x) {
+  return x;
+}
+
 var x = 1;
 
 var xx = 1;
 
 exports.x = x;
 exports.xx = xx;
+exports.foo = foo;
 /* No side effect */
diff --git a/jscomp/test/Coercion.res b/jscomp/test/Coercion.res
index 76e0eb8700..d7c89f7b54 100644
--- a/jscomp/test/Coercion.res
+++ b/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 }