Port 10404 : optimize emit default constructor for subclass (#10598)

* Port 10404 : optimize emit default constructor for subclass

* Address PR

Author: yuit

src/compiler/transformers/ts.ts

@@ -821,7 +821,7 @@ namespace ts {
                 return visitEachChild(constructor, visitor, context);
             }
 
-            const parameters = transformConstructorParameters(constructor, hasExtendsClause);
+            const parameters = transformConstructorParameters(constructor);
             const body = transformConstructorBody(node, constructor, hasExtendsClause, parameters);
 
             //  constructor(${parameters}) {
@@ -848,10 +848,25 @@ namespace ts {
          * @param constructor The constructor declaration.
          * @param hasExtendsClause A value indicating whether the class has an extends clause.
          */
-        function transformConstructorParameters(constructor: ConstructorDeclaration, hasExtendsClause: boolean) {
+        function transformConstructorParameters(constructor: ConstructorDeclaration) {
+            // The ES2015 spec specifies in 14.5.14. Runtime Semantics: ClassDefinitionEvaluation:
+            // If constructor is empty, then
+            //     If ClassHeritag_eopt is present and protoParent is not null, then
+            //          Let constructor be the result of parsing the source text
+            //              constructor(...args) { super (...args);}
+            //          using the syntactic grammar with the goal symbol MethodDefinition[~Yield].
+            //      Else,
+            //           Let constructor be the result of parsing the source text
+            //               constructor( ){ }
+            //           using the syntactic grammar with the goal symbol MethodDefinition[~Yield].
+            //
+            // While we could emit the '...args' rest parameter, certain later tools in the pipeline might
+            // downlevel the '...args' portion less efficiently by naively copying the contents of 'arguments' to an array.
+            // Instead, we'll avoid using a rest parameter and spread into the super call as
+            // 'super(...arguments)' instead of 'super(...args)', as you can see in "transformConstructorBody".
             return constructor
                 ? visitNodes(constructor.parameters, visitor, isParameter)
-                : hasExtendsClause ? [createRestParameter("args")] : [];
+                : <ParameterDeclaration[]>[];
         }
 
         /**
@@ -889,18 +904,16 @@ namespace ts {
                 addRange(statements, map(propertyAssignments, transformParameterWithPropertyAssignment));
             }
             else if (hasExtendsClause) {
-                Debug.assert(parameters.length === 1 && isIdentifier(parameters[0].name));
-
                 // Add a synthetic `super` call:
                 //
-                //  super(...args);
+                //  super(...arguments);
                 //
                 statements.push(
                     createStatement(
                         createCall(
                             createSuper(),
                             /*typeArguments*/ undefined,
-                            [createSpread(<Identifier>parameters[0].name)]
+                            [createSpread(createIdentifier("arguments"))]
                         )
                     )
                 );

tests/baselines/reference/classExpressionES63.js

@@ -12,13 +12,13 @@ let C = class extends class extends class {
         this.a = 1;
     }
 } {
-    constructor(...args) {
-        super(...args);
+    constructor() {
+        super(...arguments);
         this.b = 2;
     }
 } {
-    constructor(...args) {
-        super(...args);
+    constructor() {
+        super(...arguments);
         this.c = 3;
     }
 };

tests/baselines/reference/emitClassDeclarationWithPropertyAssignmentInES6.js

@@ -37,8 +37,8 @@ class D {
     }
 }
 class E extends D {
-    constructor(...args) {
-        super(...args);
+    constructor() {
+        super(...arguments);
         this.z = true;
     }
 }

tests/baselines/reference/sourceMapValidationClassWithDefaultConstructorAndExtendsClause.js.map

@@ -82,27 +82,20 @@ sourceFile:sourceMapValidationClassWithDefaultConstructorAndExtendsClause.ts
 1 >Emitted(13, 5) Source(4, 1) + SourceIndex(0)
 ---
 >>>        _super.apply(this, arguments);
-1->^^^^^^^^
-2 >        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-1->class Greeter extends 
-2 >        AbstractGreeter
-1->Emitted(14, 9) Source(4, 23) + SourceIndex(0)
-2 >Emitted(14, 39) Source(4, 38) + SourceIndex(0)
----
 >>>        this.a = 10;
-1 >^^^^^^^^
+1->^^^^^^^^
 2 >        ^^^^^^
 3 >              ^^^
 4 >                 ^^
 5 >                   ^
 6 >                    ^^^^^^^^->
-1 > {
+1->class Greeter extends AbstractGreeter {
   >    public 
 2 >        a
 3 >               = 
 4 >                 10
 5 >                   ;
-1 >Emitted(15, 9) Source(5, 12) + SourceIndex(0)
+1->Emitted(15, 9) Source(5, 12) + SourceIndex(0)
 2 >Emitted(15, 15) Source(5, 13) + SourceIndex(0)
 3 >Emitted(15, 18) Source(5, 16) + SourceIndex(0)
 4 >Emitted(15, 20) Source(5, 18) + SourceIndex(0)