From fb669962a0300d3a00f1f1a2ecc104b6f9c51267 Mon Sep 17 00:00:00 2001
From: William Chargin <wchargin@gmail.com>
Date: Mon, 22 Jun 2020 10:42:36 -0700
Subject: [PATCH] combo: make `JsonObject` arrays/objects covariant (#1885)

Summary:
This is more general: a mutable type is a subtype of its corresponding
read-only type. Using the covariant form lets structures like `string[]`
be subtypes of `JsonObject`, as `T[]` is not (in general) a subtype of
`(T | U)[]` but is a subtype of `$ReadOnlyArray<T | U>`.

Test Plan:
Demonstration type-level tests added.

wchargin-branch: jsonobject-readonly
---
 src/util/combo.js      | 4 ++--
 src/util/combo.test.js | 8 ++++++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/util/combo.js b/src/util/combo.js
index 4bb73e6..00b5ffe 100644
--- a/src/util/combo.js
+++ b/src/util/combo.js
@@ -8,8 +8,8 @@ export type JsonObject =
   | number
   | boolean
   | null
-  | JsonObject[]
-  | {[string]: JsonObject};
+  | $ReadOnlyArray<JsonObject>
+  | {+[string]: JsonObject};
 
 export type ParseResult<+T> =
   | {|+ok: true, +value: T|}
diff --git a/src/util/combo.test.js b/src/util/combo.test.js
index 1852c20..fec3418 100644
--- a/src/util/combo.test.js
+++ b/src/util/combo.test.js
@@ -3,6 +3,14 @@
 import * as C from "./combo";
 
 describe("src/util/combo", () => {
+  describe("type JsonObject", () => {
+    it("includes compound structures of strict subtypes", () => {
+      // (requires covariance in the array/object clauses)
+      (x: string[]): C.JsonObject => x;
+      (x: {[string]: number}): C.JsonObject => x;
+    });
+  });
+
   describe("primitives", () => {
     describe("string", () => {
       it("accepts strings", () => {