From 6394c732090efdf7798e0c8dc44c46583b4131c3 Mon Sep 17 00:00:00 2001
From: fisker Cheung <lionkay@gmail.com>
Date: Thu, 9 Jul 2026 12:57:47 +0800
Subject: [PATCH] Align empty module declaration with TS (#19568)

---
 changelog_unreleased/flow/19568.md                | 14 ++++++++++++++
 src/language-js/print/block.js                    |  1 +
 .../coverage/__snapshots__/format.test.js.snap    | 15 +++++----------
 .../__snapshots__/format.test.js.snap             |  6 ++----
 .../__snapshots__/format.test.js.snap             | 14 ++++++++++++++
 .../declare/declare-module-empty/empty.ts         |  1 +
 .../declare/declare-module-empty/format.test.js   |  1 +
 7 files changed, 38 insertions(+), 14 deletions(-)
 create mode 100644 changelog_unreleased/flow/19568.md
 create mode 100644 tests/format/typescript/declare/declare-module-empty/__snapshots__/format.test.js.snap
 create mode 100644 tests/format/typescript/declare/declare-module-empty/empty.ts
 create mode 100644 tests/format/typescript/declare/declare-module-empty/format.test.js

diff --git a/changelog_unreleased/flow/19568.md b/changelog_unreleased/flow/19568.md
new file mode 100644
index 000000000..46ba1b247
--- /dev/null
+++ b/changelog_unreleased/flow/19568.md
@@ -0,0 +1,14 @@
+#### Stop enforcing empty module declaration to break (#19568 by @fisker)
+
+<!-- prettier-ignore -->
+```flow
+// Input
+declare module "foo" {}
+
+// Prettier stable
+declare module "foo" {
+}
+
+// Prettier main
+declare module "foo" {}
+```
diff --git a/src/language-js/print/block.js b/src/language-js/print/block.js
index 74d91620c..0e9886f92 100644
--- a/src/language-js/print/block.js
+++ b/src/language-js/print/block.js
@@ -48,6 +48,7 @@ function printBlock(path, options, print) {
       parent.type === "ModuleExpression" ||
       (parent.type === "CatchClause" && !parentParent.finalizer) ||
       parent.type === "TSModuleDeclaration" ||
+      parent.type === "DeclareModule" ||
       parent.type === "MatchStatementCase" ||
       node.type === "StaticBlock"
     )) {
diff --git a/tests/format/flow/flow-repo/coverage/__snapshots__/format.test.js.snap b/tests/format/flow/flow-repo/coverage/__snapshots__/format.test.js.snap
index 14e909f53..c1b4762b5 100644
--- a/tests/format/flow/flow-repo/coverage/__snapshots__/format.test.js.snap
+++ b/tests/format/flow/flow-repo/coverage/__snapshots__/format.test.js.snap
@@ -22,11 +22,9 @@ declare module bar {
 // that we implicitly assume in type-at-pos and coverage implementations. In
 // particular, when unchecked it causes a crash with coverage --color.
 
-declare module foo {
-}
+declare module foo {}
 
-declare module bar {
-}
+declare module bar {}
 
 // TODO
 
@@ -46,8 +44,7 @@ declare module foo {
 =====================================output=====================================
 // check coverage of declare module
 
-declare module foo {
-}
+declare module foo {}
 
 ================================================================================
 `;
@@ -92,11 +89,9 @@ declare class qux {
 // that we implicitly assume in type-at-pos and coverage implementations. In
 // particular, when unchecked it causes non-termination with coverage --color.
 
-declare module foo {
-}
+declare module foo {}
 
-declare module bar {
-}
+declare module bar {}
 
 // TODO
 
diff --git a/tests/format/flow/type-declarations/__snapshots__/format.test.js.snap b/tests/format/flow/type-declarations/__snapshots__/format.test.js.snap
index db4a8766e..ddf787553 100644
--- a/tests/format/flow/type-declarations/__snapshots__/format.test.js.snap
+++ b/tests/format/flow/type-declarations/__snapshots__/format.test.js.snap
@@ -44,8 +44,7 @@ declare module "foo" {
   declare var bar: string
 }
 
-declare module baz {
-}
+declare module baz {}
 
 ================================================================================
 `;
@@ -66,8 +65,7 @@ declare module "foo" {
   declare var bar: string;
 }
 
-declare module baz {
-}
+declare module baz {}
 
 ================================================================================
 `;
diff --git a/tests/format/typescript/declare/declare-module-empty/__snapshots__/format.test.js.snap b/tests/format/typescript/declare/declare-module-empty/__snapshots__/format.test.js.snap
new file mode 100644
index 000000000..04c380b7a
--- /dev/null
+++ b/tests/format/typescript/declare/declare-module-empty/__snapshots__/format.test.js.snap
@@ -0,0 +1,14 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`empty.ts format 1`] = `
+====================================options=====================================
+parsers: ["typescript", "flow"]
+                                                      printWidth: 80 (default) |
+=====================================input======================================
+declare module "foo" {}
+
+=====================================output=====================================
+declare module "foo" {}
+
+================================================================================
+`;
diff --git a/tests/format/typescript/declare/declare-module-empty/empty.ts b/tests/format/typescript/declare/declare-module-empty/empty.ts
new file mode 100644
index 000000000..3945956c2
--- /dev/null
+++ b/tests/format/typescript/declare/declare-module-empty/empty.ts
@@ -0,0 +1 @@
+declare module "foo" {}
diff --git a/tests/format/typescript/declare/declare-module-empty/format.test.js b/tests/format/typescript/declare/declare-module-empty/format.test.js
new file mode 100644
index 000000000..0e7a632d1
--- /dev/null
+++ b/tests/format/typescript/declare/declare-module-empty/format.test.js
@@ -0,0 +1 @@
+runFormatTest(import.meta, ["typescript", "flow"]);