From a8eb09553a6b34189f607325a8336bfc2a90a4ab Mon Sep 17 00:00:00 2001
From: Christopher Chedeau <vjeuxx@gmail.com>
Date: Wed, 18 Jan 2017 09:42:20 -0800
Subject: [PATCH] Do not output trailing commas with rest arguments (#283)

It turns out that this is not valid by the spec!
---
 src/printer.js                              |  5 +++-
 tests/rest/__snapshots__/jsfmt.spec.js.snap | 27 +++++++++++++++++++++
 tests/rest/jsfmt.spec.js                    |  1 +
 tests/rest/trailing-commas.js               | 11 +++++++++
 4 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 tests/rest/__snapshots__/jsfmt.spec.js.snap
 create mode 100644 tests/rest/jsfmt.spec.js
 create mode 100644 tests/rest/trailing-commas.js

diff --git a/src/printer.js b/src/printer.js
index 78ed5c08..fbd6c54d 100644
--- a/src/printer.js
+++ b/src/printer.js
@@ -1760,13 +1760,16 @@ function printFunctionParams(path, print, options) {
     return "()";
   }
 
+  const lastParam = util.getLast(path.getValue().params);
+  const canHaveTrailingComma = lastParam.type !== "RestElement" && !fun.rest;
+
   return concat([
     "(",
     indent(
       options.tabWidth,
       concat([ softline, join(concat([ ",", line ]), printed) ])
     ),
-    ifBreak(options.trailingComma ? "," : ""),
+    ifBreak(canHaveTrailingComma && options.trailingComma ? "," : ""),
     softline,
     ")"
   ]);
diff --git a/tests/rest/__snapshots__/jsfmt.spec.js.snap b/tests/rest/__snapshots__/jsfmt.spec.js.snap
new file mode 100644
index 00000000..70b59847
--- /dev/null
+++ b/tests/rest/__snapshots__/jsfmt.spec.js.snap
@@ -0,0 +1,27 @@
+exports[`test trailing-commas.js 1`] = `
+"declare class C {
+  f(
+    superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
+    ...args
+  ): void,
+}
+
+function f(
+  superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
+  ...args
+) {}
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+declare class C {
+  f(
+    superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
+    ...args
+  ): void,
+}
+
+function f(
+  superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
+  ...args
+) {
+}
+"
+`;
diff --git a/tests/rest/jsfmt.spec.js b/tests/rest/jsfmt.spec.js
new file mode 100644
index 00000000..5495c026
--- /dev/null
+++ b/tests/rest/jsfmt.spec.js
@@ -0,0 +1 @@
+run_spec(__dirname, {trailingComma: true});
diff --git a/tests/rest/trailing-commas.js b/tests/rest/trailing-commas.js
new file mode 100644
index 00000000..39ebdccd
--- /dev/null
+++ b/tests/rest/trailing-commas.js
@@ -0,0 +1,11 @@
+declare class C {
+  f(
+    superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
+    ...args
+  ): void,
+}
+
+function f(
+  superSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperSuperLong,
+  ...args
+) {}