Support readonly as Flow variance annotation (#19022)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: fisker Cheung <lionkay@gmail.com>

changelog_unreleased/flow/19022.md

@@ -0,0 +1,19 @@
+#### Support `readonly` as a variance annotation (#19022 by @marcoww6)
+
+Flow now accepts `readonly` as a property variance annotation, equivalent to `+` (covariant/read-only).
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+type T = {
+  readonly foo: string,
+};
+
+// Prettier stable
+SyntaxError
+
+// Prettier main
+type T = {
+  readonly foo: string,
+};
+```

src/language-js/print/flow.js

@@ -218,8 +218,8 @@ function printFlow(path, options, print, args) {
       return ["?", print("typeAnnotation")];
     case "Variance": {
       const { kind } = node;
-      assert.ok(kind === "plus" || kind === "minus");
-      return kind === "plus" ? "+" : "-";
+      assert.ok(kind === "plus" || kind === "minus" || kind === "readonly");
+      return kind === "plus" ? "+" : kind === "minus" ? "-" : `${kind} `;
     }
     case "KeyofTypeAnnotation":
       return ["keyof ", print("argument")];

src/language-js/types/flow-estree.d.ts

@@ -1200,7 +1200,7 @@ export type TypeAnnotationType =
 
 export interface Variance extends BaseNode {
   type: "Variance";
-  kind: "plus" | "minus";
+  kind: "plus" | "minus" | "readonly";
 }
 
 interface BaseTypeAlias extends BaseNode {

tests/format/flow/variance/__snapshots__/format.test.js.snap

@@ -9,10 +9,40 @@ class Route {
   static +param: T;
 }
 
+class ReadonlyRoute {
+    static   readonly   param: T;
+}
+
+type ReadonlyObj = {
+    readonly   foo:   string,
+  readonly [string]:   unknown
+};
+
+type ReadonlyTuple = [  readonly   label:   number  ];
+
+type ReadonlyInterface = interface {
+      readonly   prop:   string
+};
+
 =====================================output=====================================
 class Route {
   static +param: T;
 }
 
+class ReadonlyRoute {
+  static readonly param: T;
+}
+
+type ReadonlyObj = {
+  readonly foo: string,
+  readonly [string]: unknown,
+};
+
+type ReadonlyTuple = [readonly label: number];
+
+type ReadonlyInterface = interface {
+  readonly prop: string,
+};
+
 ================================================================================
 `;

tests/format/flow/variance/variance.js

@@ -1,3 +1,18 @@
 class Route {
   static +param: T;
 }
+
+class ReadonlyRoute {
+    static   readonly   param: T;
+}
+
+type ReadonlyObj = {
+    readonly   foo:   string,
+  readonly [string]:   unknown
+};
+
+type ReadonlyTuple = [  readonly   label:   number  ];
+
+type ReadonlyInterface = interface {
+      readonly   prop:   string
+};