From de80aefece3049ffd5986eef0a968d13e8c21f81 Mon Sep 17 00:00:00 2001
From: Ryan Marsh <ryan.marsh@mac.com>
Date: Tue, 30 Oct 2018 00:49:11 -0500
Subject: [PATCH] fix single-arg async arrows when retainLines=true (#8868)

---
 .../babel-generator/src/generators/methods.js  | 18 +++++++++++++++++-
 .../input.js                                   | 11 +++++++++++
 .../options.json                               |  3 +++
 .../output.js                                  | 11 +++++++++++
 4 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/input.js
 create mode 100644 packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/options.json
 create mode 100644 packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/output.js

diff --git a/packages/babel-generator/src/generators/methods.js b/packages/babel-generator/src/generators/methods.js
index 5dc17e065a20..9d9f68733380 100644
--- a/packages/babel-generator/src/generators/methods.js
+++ b/packages/babel-generator/src/generators/methods.js
@@ -111,7 +111,23 @@ export function ArrowFunctionExpression(node: Object) {
     t.isIdentifier(firstParam) &&
     !hasTypes(node, firstParam)
   ) {
-    this.print(firstParam, node);
+    if (
+      this.format.retainLines &&
+      node.loc.start.line < node.body.loc.start.line
+    ) {
+      this.token("(");
+      if (firstParam.loc.start.line > node.loc.start.line) {
+        this.indent();
+        this.print(firstParam, node);
+        this.dedent();
+        this._catchUp("start", node.body.loc);
+      } else {
+        this.print(firstParam, node);
+      }
+      this.token(")");
+    } else {
+      this.print(firstParam, node);
+    }
   } else {
     this._params(node);
   }
diff --git a/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/input.js b/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/input.js
new file mode 100644
index 000000000000..e409b79f575f
--- /dev/null
+++ b/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/input.js
@@ -0,0 +1,11 @@
+var fn = async (
+    arg
+) => {}
+
+async (x)
+=> {}
+
+async x => {}
+
+async (x
+) => {};
\ No newline at end of file
diff --git a/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/options.json b/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/options.json
new file mode 100644
index 000000000000..97925bbcb61b
--- /dev/null
+++ b/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/options.json
@@ -0,0 +1,3 @@
+{
+  "retainLines": true
+}
diff --git a/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/output.js b/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/output.js
new file mode 100644
index 000000000000..6bb2ce5093e8
--- /dev/null
+++ b/packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/output.js
@@ -0,0 +1,11 @@
+var fn = async (
+  arg
+) => {};
+
+async (x) =>
+{};
+
+async x => {};
+
+async (x) =>
+{};
\ No newline at end of file