Better error for disallowed trailing commas/parameters after rest ele…

…ments (#9046)

* handle disordered rest parameter in function expressions

* remove spaces [lint]

* polish function parameters validation

* add test with arrow function and comma after rest parameter [babel-parser]

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

@@ -1113,11 +1113,13 @@ export default class ExpressionParser extends LValParser {
           ),
         );
 
-        if (this.match(tt.comma) && this.lookahead().type === tt.parenR) {
-          this.raise(
-            this.state.start,
-            "A trailing comma is not permitted after the rest element",
-          );
+        if (this.match(tt.comma)) {
+          const nextTokenType = this.lookahead().type;
+          const errorMessage =
+            nextTokenType === tt.parenR
+              ? "A trailing comma is not permitted after the rest element"
+              : "Rest parameter must be last formal parameter";
+          this.raise(this.state.start, errorMessage);
         }
 
         break;

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

@@ -258,7 +258,20 @@ 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.match(tt.comma)
+        ) {
+          const nextTokenType = this.lookahead().type;
+          const errorMessage =
+            nextTokenType === tt.parenR
+              ? "A trailing comma is not permitted after the rest element"
+              : "Rest parameter must be last formal parameter";
+          this.raise(this.state.start, errorMessage);
+        } 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/comma-after-rest-param/input.js

@@ -0,0 +1 @@
+(...rest,) => {}

packages/babel-parser/test/fixtures/es2015/arrow-functions/comma-after-rest-param/options.json

@@ -0,0 +1,3 @@
+{
+    "throws": "A trailing comma is not permitted after the rest element (1:8)"
+}

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)"
 }