Simplify emit for syntactic 'extends null' case

src/compiler/transformers/es2015.ts

@@ -925,7 +925,10 @@ namespace ts {
 
             }
 
-            const superCaptureStatus = declareOrCaptureOrReturnThisForConstructorIfNeeded(statements, constructor, !!extendsClauseElement, hasSynthesizedSuper, statementOffset);
+            // determine whether the class is known syntactically to be a derived class (e.g. a
+            // class that extends a value that is not syntactically known to be `null`).
+            const isDerivedClass = extendsClauseElement && skipOuterExpressions(extendsClauseElement.expression).kind !== SyntaxKind.NullKeyword;
+            const superCaptureStatus = declareOrCaptureOrReturnThisForConstructorIfNeeded(statements, constructor, isDerivedClass, hasSynthesizedSuper, statementOffset);
 
             // The last statement expression was replaced. Skip it.
             if (superCaptureStatus === SuperCaptureResult.ReplaceSuperCapture || superCaptureStatus === SuperCaptureResult.ReplaceWithReturn) {
@@ -942,7 +945,7 @@ namespace ts {
 
             // Return `_this` unless we're sure enough that it would be pointless to add a return statement.
             // If there's a constructor that we can tell returns in enough places, then we *do not* want to add a return.
-            if (extendsClauseElement
+            if (isDerivedClass
                 && superCaptureStatus !== SuperCaptureResult.ReplaceWithReturn
                 && !(constructor && isSufficientlyCoveredByReturnStatements(constructor.body))) {
                 statements.push(
@@ -1011,11 +1014,11 @@ namespace ts {
         function declareOrCaptureOrReturnThisForConstructorIfNeeded(
                     statements: Statement[],
                     ctor: ConstructorDeclaration | undefined,
-                    hasExtendsClause: boolean,
+                    isDerivedClass: boolean,
                     hasSynthesizedSuper: boolean,
                     statementOffset: number) {
             // If this isn't a derived class, just capture 'this' for arrow functions if necessary.
-            if (!hasExtendsClause) {
+            if (!isDerivedClass) {
                 if (ctor) {
                     addCaptureThisForNodeIfNeeded(statements, ctor);
                 }

tests/baselines/reference/classExtendingNull.js

@@ -17,14 +17,12 @@ var __extends = (this && this.__extends) || (function () {
 var C1 = (function (_super) {
     __extends(C1, _super);
     function C1() {
-        return _super !== null && _super.apply(this, arguments) || this;
     }
     return C1;
 }(null));
 var C2 = (function (_super) {
     __extends(C2, _super);
     function C2() {
-        return _super !== null && _super.apply(this, arguments) || this;
     }
     return C2;
 }((null)));

tests/baselines/reference/classExtendingPrimitive.js

@@ -70,7 +70,6 @@ var C5 = (function (_super) {
 var C5a = (function (_super) {
     __extends(C5a, _super);
     function C5a() {
-        return _super !== null && _super.apply(this, arguments) || this;
     }
     return C5a;
 }(null));

tests/baselines/reference/classExtendingPrimitive2.js

@@ -25,7 +25,6 @@ void {};
 var C5a = (function (_super) {
     __extends(C5a, _super);
     function C5a() {
-        return _super !== null && _super.apply(this, arguments) || this;
     }
     return C5a;
 }(null));

tests/baselines/reference/classExtendsNull.js

@@ -26,7 +26,7 @@ var __extends = (this && this.__extends) || (function () {
 var C = (function (_super) {
     __extends(C, _super);
     function C() {
-        var _this = _super.call(this) || this;
+        _this = _super.call(this) || this;
         return Object.create(null);
     }
     return C;
@@ -34,7 +34,6 @@ var C = (function (_super) {
 var D = (function (_super) {
     __extends(D, _super);
     function D() {
-        var _this = this;
         return Object.create(null);
     }
     return D;

tests/baselines/reference/declFileClassExtendsNull.js

@@ -17,7 +17,6 @@ var __extends = (this && this.__extends) || (function () {
 var ExtendsNull = (function (_super) {
     __extends(ExtendsNull, _super);
     function ExtendsNull() {
-        return _super !== null && _super.apply(this, arguments) || this;
     }
     return ExtendsNull;
 }(null));

tests/baselines/reference/superCallBeforeThisAccessing4.js

@@ -29,19 +29,16 @@ var __extends = (this && this.__extends) || (function () {
 var D = (function (_super) {
     __extends(D, _super);
     function D() {
-        var _this = this;
-        _this._t;
+        this._t;
         _this = _super.call(this) || this;
-        return _this;
     }
     return D;
 }(null));
 var E = (function (_super) {
     __extends(E, _super);
     function E() {
-        var _this = _super.call(this) || this;
-        _this._t;
-        return _this;
+        _this = _super.call(this) || this;
+        this._t;
     }
     return E;
 }(null));

tests/baselines/reference/superCallBeforeThisAccessing5.js

@@ -21,9 +21,7 @@ var __extends = (this && this.__extends) || (function () {
 var D = (function (_super) {
     __extends(D, _super);
     function D() {
-        var _this = this;
-        _this._t; // No error
-        return _this;
+        this._t; // No error
     }
     return D;
 }(null));