From 74923fa8316cd78f8461e77290915155efbcd714 Mon Sep 17 00:00:00 2001
From: fisker <lionkay@gmail.com>
Date: Fri, 26 Jun 2026 20:40:05 +0800
Subject: [PATCH] Fix variance annotation

---
 changelog_unreleased/flow/19022.md | 19 +++++++++++++++++++
 src/language-js/print/flow.js      | 11 +++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 changelog_unreleased/flow/19022.md

diff --git a/changelog_unreleased/flow/19022.md b/changelog_unreleased/flow/19022.md
new file mode 100644
index 000000000..7b26a5c17
--- /dev/null
+++ b/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,
+};
+```
\ No newline at end of file
diff --git a/src/language-js/print/flow.js b/src/language-js/print/flow.js
index 24230abfb..836e9bd48 100644
--- a/src/language-js/print/flow.js
+++ b/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")];