Do not transform the emit of function with rest parameter unless declared in AST

Fixes #7749

Author: sheetalkamat

src/compiler/checker.ts

@@ -13811,7 +13811,7 @@ namespace ts {
 
         function checkCollisionWithArgumentsInGeneratedCode(node: SignatureDeclaration) {
             // no rest parameters \ declaration context \ overload - no codegen impact
-            if (!hasRestParameter(node) || isInAmbientContext(node) || nodeIsMissing((<FunctionLikeDeclaration>node).body)) {
+            if (!hasDeclaredRestParameter(node) || isInAmbientContext(node) || nodeIsMissing((<FunctionLikeDeclaration>node).body)) {
                 return;
             }
 

src/compiler/emitter.ts

@@ -4487,7 +4487,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
             }
 
             function emitRestParameter(node: FunctionLikeDeclaration) {
-                if (languageVersion < ScriptTarget.ES6 && hasRestParameter(node)) {
+                if (languageVersion < ScriptTarget.ES6 && hasDeclaredRestParameter(node)) {
                     const restIndex = node.parameters.length - 1;
                     const restParam = node.parameters[restIndex];
 
@@ -4644,7 +4644,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
                 if (node) {
                     const parameters = node.parameters;
                     const skipCount = node.parameters.length && (<Identifier>node.parameters[0].name).text === "this" ? 1 : 0;
-                    const omitCount = languageVersion < ScriptTarget.ES6 && hasRestParameter(node) ? 1 : 0;
+                    const omitCount = languageVersion < ScriptTarget.ES6 && hasDeclaredRestParameter(node) ? 1 : 0;
                     emitList(parameters, skipCount, parameters.length - omitCount - skipCount, /*multiLine*/ false, /*trailingComma*/ false);
                 }
                 write(")");

src/compiler/utilities.ts

@@ -1377,23 +1377,26 @@ namespace ts {
         return isRestParameter(lastOrUndefined(s.parameters));
     }
 
-    export function isRestParameter(node: ParameterDeclaration) {
-        if (node) {
-            if (node.flags & NodeFlags.JavaScriptFile) {
-                if (node.type && node.type.kind === SyntaxKind.JSDocVariadicType) {
-                    return true;
-                }
+    export function hasDeclaredRestParameter(s: SignatureDeclaration): boolean {
+        return isDeclaredRestParam(lastOrUndefined(s.parameters));
+    }
 
-                const paramTag = getCorrespondingJSDocParameterTag(node);
-                if (paramTag && paramTag.typeExpression) {
-                    return paramTag.typeExpression.type.kind === SyntaxKind.JSDocVariadicType;
-                }
+    export function isRestParameter(node: ParameterDeclaration) {
+        if (node && (node.flags & NodeFlags.JavaScriptFile)) {
+            if (node.type && node.type.kind === SyntaxKind.JSDocVariadicType) {
+                return true;
             }
 
-            return node.dotDotDotToken !== undefined;
+            const paramTag = getCorrespondingJSDocParameterTag(node);
+            if (paramTag && paramTag.typeExpression) {
+                return paramTag.typeExpression.type.kind === SyntaxKind.JSDocVariadicType;
+            }
         }
+        return isDeclaredRestParam(node);
+    }
 
-        return false;
+    export function isDeclaredRestParam(node: ParameterDeclaration) {
+        return node && node.dotDotDotToken !== undefined;
     }
 
     export function isLiteralKind(kind: SyntaxKind): boolean {

tests/baselines/reference/jsFileCompilationRestParamJsDocFunction.js

@@ -36,11 +36,7 @@ define("_apply", ["require", "exports"], function (require, exports) {
      * @param {...*} args The arguments to invoke `func` with.
      * @returns {*} Returns the result of `func`.
      */
-    function apply(func, thisArg) {
-        var args = [];
-        for (var _i = 2; _i < arguments.length; _i++) {
-            args[_i - 2] = arguments[_i];
-        }
+    function apply(func, thisArg, args) {
         var length = args.length;
         switch (length) {
             case 0: return func.call(thisArg);