Fix variance annotation

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

@@ -201,8 +201,15 @@ function printFlow(path, options, print) {
       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" ||
+          kind === "writeonly" ||
+          kind === "in" ||
+          kind === "out",
+      );
+      return kind === "plus" ? "+" : kind === "minus" ? "-" : `${kind} `;
     }
     case "KeyofTypeAnnotation":
       return ["keyof ", print("argument")];