fix single-arg async arrows when retainLines=true (#8868)

packages/babel-generator/src/generators/methods.js

@@ -111,7 +111,23 @@ export function ArrowFunctionExpression(node: Object) {
     t.isIdentifier(firstParam) &&
     !hasTypes(node, firstParam)
   ) {
+    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);
   }

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
+) => {};

packages/babel-generator/test/fixtures/edgecase/single-arg-async-arrow-with-retainlines/options.json

@@ -0,0 +1,3 @@
+{
+  "retainLines": true
+}

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) =>
+{};