a better error message for disallowed trailing commas/additional parameters after rest elements in function params

packages/babel-parser/src/parser/expression.js

@@ -1120,6 +1120,17 @@ export default class ExpressionParser extends LValParser {
           );
         }
 
+        if (
+          this.match(tt.comma) &&
+          (this.lookahead().type === tt.name ||
+            this.lookahead().type === tt.ellipsis)
+        ) {
+          this.raise(
+            this.state.start,
+            "Rest parameter must be last formal parameter",
+          );
+        }
+
         break;
       } else {
         exprList.push(

packages/babel-parser/src/parser/lval.js

@@ -258,7 +258,19 @@ export default class LValParser extends NodeUtils {
         break;
       } else if (this.match(tt.ellipsis)) {
         elts.push(this.parseAssignableListItemTypes(this.parseRest()));
-        this.expect(close);
+        if (
+          this.state.inFunction &&
+          this.state.inParameters &&
+          (this.lookahead().type === tt.name ||
+            this.lookahead().type === tt.ellipsis)
+        ) {
+          this.raise(
+            this.state.start,
+            "Rest parameter must be last formal parameter",
+          );
+        } else {
+          this.expect(close);
+        }
         break;
       } else {
         const decorators = [];

packages/babel-parser/test/fixtures/core/uncategorised/396/options.json

@@ -1,3 +1,3 @@
 {
-  "throws": "Unexpected token, expected \")\" (1:18)"
+  "throws": "Rest parameter must be last formal parameter (1:18)"
 }

packages/babel-parser/test/fixtures/core/uncategorised/555/input.js

@@ -0,0 +1,6 @@
+function foo ( 
+    first,
+    ...second,
+    third,
+) {
+};

packages/babel-parser/test/fixtures/core/uncategorised/555/options.json

@@ -0,0 +1,4 @@
+{
+    "throws": "Rest parameter must be last formal parameter (3:13)"
+  }
+

packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-rest-in-params/input.js

@@ -0,0 +1,5 @@
+(
+    first,
+    ...second,
+    third
+) => {};

packages/babel-parser/test/fixtures/es2015/arrow-functions/invalid-rest-in-params/options.json

@@ -0,0 +1,4 @@
+{
+    "throws": "Rest parameter must be last formal parameter (3:13)"
+}
+

packages/babel-parser/test/fixtures/es2015/uncategorised/277/options.json

@@ -1,3 +1,3 @@
 {
-  "throws": "Unexpected token, expected \")\" (1:18)"
+  "throws": "Rest parameter must be last formal parameter (1:18)"
 }

packages/babel-parser/test/fixtures/es2015/uncategorised/283/options.json

@@ -1,3 +1,3 @@
 {
-  "throws": "Unexpected token, expected \")\" (1:5)"
+  "throws": "Rest parameter must be last formal parameter (1:5)"
 }

packages/babel-parser/test/fixtures/esprima/es2015-arrow-function/arrow-with-multiple-rest/options.json

@@ -1,3 +1,3 @@
 {
-  "throws": "Unexpected token, expected \")\" (1:5)"
+  "throws": "Rest parameter must be last formal parameter (1:5)"
 }

packages/babel-parser/test/fixtures/esprima/invalid-syntax/migrated_0258/options.json

@@ -1,3 +1,3 @@
 {
-  "throws": "Unexpected token, expected \")\" (1:18)"
+  "throws": "Rest parameter must be last formal parameter (1:18)"
 }