Report a semantic error for an arrow function with a "this" parameter.

Fixes microsoft#9744.

Author: mattmccutchen

src/compiler/checker.ts

@@ -21857,6 +21857,9 @@ namespace ts {
                 if (func.kind === SyntaxKind.Constructor || func.kind === SyntaxKind.ConstructSignature || func.kind === SyntaxKind.ConstructorType) {
                     error(node, Diagnostics.A_constructor_cannot_have_a_this_parameter);
                 }
+                if (func.kind === SyntaxKind.ArrowFunction) {
+                    error(node, Diagnostics.An_arrow_function_cannot_have_a_this_parameter);
+                }
             }
 
             // Only check rest parameter type if it's not a binding pattern. Since binding patterns are

src/compiler/diagnosticMessages.json

@@ -2417,6 +2417,10 @@
         "category": "Error",
         "code": 2729
     },
+    "An arrow function cannot have a 'this' parameter.": {
+        "category": "Error",
+        "code": 2730
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

src/compiler/parser.ts

@@ -3537,8 +3537,9 @@ namespace ts {
                 }
 
                 // If we had "(" followed by something that's not an identifier,
-                // then this definitely doesn't look like a lambda.
-                if (!isIdentifier()) {
+                // then this definitely doesn't look like a lambda.  "this" is not
+                // valid, but we want to parse it and then give a semantic error.
+                if (!isIdentifier() && second !== SyntaxKind.ThisKeyword) {
                     return Tristate.False;
                 }
 

tests/baselines/reference/thisTypeInFunctionsNegative.errors.txt

@@ -93,12 +93,13 @@ tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(172,39): e
 tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(172,40): error TS1128: Declaration or statement expected.
 tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(172,42): error TS2693: 'number' only refers to a type, but is being used as a value here.
 tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(172,49): error TS1005: ';' expected.
-tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(175,29): error TS2304: Cannot find name 'm'.
-tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(175,32): error TS1005: ';' expected.
-tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(175,35): error TS2304: Cannot find name 'm'.
+tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(175,23): error TS2730: An arrow function cannot have a 'this' parameter.
+tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(176,16): error TS2730: An arrow function cannot have a 'this' parameter.
+tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(177,19): error TS2730: An arrow function cannot have a 'this' parameter.
+tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(178,22): error TS2730: An arrow function cannot have a 'this' parameter.
 
 
-==== tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts (64 errors) ====
+==== tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts (65 errors) ====
     class C {
         n: number;
         explicitThis(this: this, m: number): number {
@@ -430,10 +431,15 @@ tests/cases/conformance/types/thisType/thisTypeInFunctionsNegative.ts(175,35): e
 
     // can't name parameters 'this' in a lambda.
     c.explicitProperty = (this, m) => m + this.n;
-                                ~
-!!! error TS2304: Cannot find name 'm'.
-                                   ~~
-!!! error TS1005: ';' expected.
-                                      ~
-!!! error TS2304: Cannot find name 'm'.
+                          ~~~~
+!!! error TS2730: An arrow function cannot have a 'this' parameter.
+    const f2 = <T>(this: {n: number}, m: number) => m + this.n;
+                   ~~~~~~~~~~~~~~~~~
+!!! error TS2730: An arrow function cannot have a 'this' parameter.
+    const f3 = async (this: {n: number}, m: number) => m + this.n;
+                      ~~~~~~~~~~~~~~~~~
+!!! error TS2730: An arrow function cannot have a 'this' parameter.
+    const f4 = async <T>(this: {n: number}, m: number) => m + this.n;
+                         ~~~~~~~~~~~~~~~~~
+!!! error TS2730: An arrow function cannot have a 'this' parameter.
 

tests/baselines/reference/thisTypeInFunctionsNegative.js

@@ -174,69 +174,60 @@ function initializer(this: C = new C()): number { return this.n; }
 
 // can't name parameters 'this' in a lambda.
 c.explicitProperty = (this, m) => m + this.n;
+const f2 = <T>(this: {n: number}, m: number) => m + this.n;
+const f3 = async (this: {n: number}, m: number) => m + this.n;
+const f4 = async <T>(this: {n: number}, m: number) => m + this.n;
 
 
 //// [thisTypeInFunctionsNegative.js]
-var __extends = (this && this.__extends) || (function () {
-    var extendStatics = function (d, b) {
-        extendStatics = Object.setPrototypeOf ||
-            ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
-            function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
-        return extendStatics(d, b);
-    }
-    return function (d, b) {
-        extendStatics(d, b);
-        function __() { this.constructor = d; }
-        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
-    };
-})();
-var _this = this;
-var C = /** @class */ (function () {
-    function C() {
-    }
-    C.prototype.explicitThis = function (m) {
+var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, generator) {
+    return new (P || (P = Promise))(function (resolve, reject) {
+        function fulfilled(value) { try { step(generator.next(value)); } catch (e) { reject(e); } }
+        function rejected(value) { try { step(generator["throw"](value)); } catch (e) { reject(e); } }
+        function step(result) { result.done ? resolve(result.value) : new P(function (resolve) { resolve(result.value); }).then(fulfilled, rejected); }
+        step((generator = generator.apply(thisArg, _arguments || [])).next());
+    });
+};
+class C {
+    explicitThis(m) {
         return this.n + m;
-    };
-    C.prototype.implicitThis = function (m) {
+    }
+    implicitThis(m) {
         return this.n + m;
-    };
-    C.prototype.explicitC = function (m) {
+    }
+    explicitC(m) {
         return this.n + m;
-    };
-    C.prototype.explicitProperty = function (m) {
+    }
+    explicitProperty(m) {
         return this.n + m;
-    };
-    C.prototype.explicitVoid = function (m) {
+    }
+    explicitVoid(m) {
         return this.n + m; // 'n' doesn't exist on type 'void'.
-    };
-    return C;
-}());
-var D = /** @class */ (function () {
-    function D() {
     }
-    D.prototype.explicitThis = function (m) {
+}
+class D {
+    explicitThis(m) {
         return this.x + m;
-    };
-    D.prototype.explicitD = function (m) {
+    }
+    explicitD(m) {
         return this.x + m;
-    };
-    return D;
-}());
-var impl = {
+    }
+}
+let impl = {
     a: 12,
-    explicitVoid1: function () {
+    explicitVoid1() {
         return this.a; // error, no 'a' in 'void'
     },
-    explicitVoid2: function () { return _this.a; },
-    explicitStructural: function () { return 12; },
-    explicitInterface: function () { return 12; },
-    explicitThis: function () {
+    explicitVoid2: () => this.a,
+    explicitStructural: () => 12,
+    explicitInterface: () => 12,
+    explicitThis() {
         return this.a;
-    }
+    },
 };
-var implExplicitStructural = impl.explicitStructural;
+let implExplicitStructural = impl.explicitStructural;
 implExplicitStructural(); // error, no 'a' in 'void'
-var implExplicitInterface = impl.explicitInterface;
+let implExplicitInterface = impl.explicitInterface;
 implExplicitInterface(); // error, no 'a' in 'void' 
 function explicitStructural(x) {
     return x + this.y;
@@ -247,15 +238,15 @@ function propertyName(x) {
 function voidThisSpecified(x) {
     return x + this.notSpecified;
 }
-var ok = { y: 12, explicitStructural: explicitStructural };
-var wrongPropertyType = { y: 'foo', explicitStructural: explicitStructural };
-var wrongPropertyName = { wrongName: 12, explicitStructural: explicitStructural };
+let ok = { y: 12, explicitStructural };
+let wrongPropertyType = { y: 'foo', explicitStructural };
+let wrongPropertyName = { wrongName: 12, explicitStructural };
 ok.f(); // not enough arguments
 ok.f('wrong type');
 ok.f(13, 'too many arguments');
 wrongPropertyType.f(13);
 wrongPropertyName.f(13);
-var c = new C();
+let c = new C();
 c.explicitC(); // not enough arguments
 c.explicitC('wrong type');
 c.explicitC(13, 'too many arguments');
@@ -269,8 +260,8 @@ c.explicitProperty(); // not enough arguments
 c.explicitProperty('wrong type 3');
 c.explicitProperty(15, 'too many arguments 3');
 // oops, this triggers contextual typing, which needs to be updated to understand that =>'s `this` is void.
-var specifiedToVoid = explicitStructural;
-var reconstructed = {
+let specifiedToVoid = explicitStructural;
+let reconstructed = {
     n: 12,
     explicitThis: c.explicitThis,
     explicitC: c.explicitC,
@@ -279,8 +270,8 @@ var reconstructed = {
 };
 ;
 // lambdas have this: void for assignability purposes (and this unbound (free) for body checking)
-var d = new D();
-var explicitXProperty;
+let d = new D();
+let explicitXProperty;
 // from differing object types
 c.explicitC = function (m) { return this.x + m; };
 c.explicitProperty = explicitXProperty;
@@ -293,64 +284,41 @@ c.explicitThis = d.explicitThis;
 c.explicitVoid = d.explicitD;
 c.explicitVoid = d.explicitThis;
 /// class-based polymorphic assignability (with inheritance!) ///
-var Base1 = /** @class */ (function () {
-    function Base1() {
-    }
-    Base1.prototype.polymorphic = function () { return this.x; };
-    Base1.prototype.explicit = function () { return this.x; };
-    Base1.explicitStatic = function () { return this.x; };
-    return Base1;
-}());
-var Derived1 = /** @class */ (function (_super) {
-    __extends(Derived1, _super);
-    function Derived1() {
-        return _super !== null && _super.apply(this, arguments) || this;
-    }
-    return Derived1;
-}(Base1));
-var Base2 = /** @class */ (function () {
-    function Base2() {
-    }
-    Base2.prototype.polymorphic = function () { return this.y; };
-    Base2.prototype.explicit = function () { return this.x; };
-    return Base2;
-}());
-var Derived2 = /** @class */ (function (_super) {
-    __extends(Derived2, _super);
-    function Derived2() {
-        return _super !== null && _super.apply(this, arguments) || this;
-    }
-    return Derived2;
-}(Base2));
-var b1 = new Base1();
-var d1 = new Derived1();
-var b2 = new Base2();
-var d2 = new Derived2();
+class Base1 {
+    polymorphic() { return this.x; }
+    explicit() { return this.x; }
+    static explicitStatic() { return this.x; }
+}
+class Derived1 extends Base1 {
+}
+class Base2 {
+    polymorphic() { return this.y; }
+    explicit() { return this.x; }
+}
+class Derived2 extends Base2 {
+}
+let b1 = new Base1();
+let d1 = new Derived1();
+let b2 = new Base2();
+let d2 = new Derived2();
 b1.polymorphic = b2.polymorphic; // error, 'this.y' not in Base1: { x }
 b1.explicit = b2.polymorphic; // error, 'y' not in Base1: { x }
 d1.explicit = b2.polymorphic; // error, 'y' not in Base1: { x }
 ////// use this-type for construction with new ////
 function VoidThis() {
 }
-var voidThis = new VoidThis();
+let voidThis = new VoidThis();
 ///// syntax-ish errors /////
-var ThisConstructor = /** @class */ (function () {
-    function ThisConstructor(n) {
+class ThisConstructor {
+    constructor(n) {
         this.n = n;
     }
-    return ThisConstructor;
-}());
+}
 var thisConstructorType;
 function notFirst(a) { return this.n; }
 ///// parse errors /////
 function modifiers() { return this.n; }
-function restParam() {
-    var  = [];
-    for (var _i = 0; _i < arguments.length; _i++) {
-        [_i] = arguments[_i];
-    }
-    return this.n;
-}
+function restParam(...) { return this.n; }
 function optional() { return this.n; }
 function decorated() { return this.n; }
 function initializer(, C) { }
@@ -360,5 +328,7 @@ number;
     return this.n;
 }
 // can't name parameters 'this' in a lambda.
-c.explicitProperty = (this, m);
-m + this.n;
+c.explicitProperty = (m) => m + this.n;
+const f2 = (m) => m + this.n;
+const f3 = (m) => __awaiter(this, void 0, void 0, function* () { return m + this.n; });
+const f4 = (m) => __awaiter(this, void 0, void 0, function* () { return m + this.n; });

tests/baselines/reference/thisTypeInFunctionsNegative.symbols

@@ -661,4 +661,30 @@ c.explicitProperty = (this, m) => m + this.n;
 >c.explicitProperty : Symbol(C.explicitProperty, Decl(thisTypeInFunctionsNegative.ts, 10, 5))
 >c : Symbol(c, Decl(thisTypeInFunctionsNegative.ts, 70, 3))
 >explicitProperty : Symbol(C.explicitProperty, Decl(thisTypeInFunctionsNegative.ts, 10, 5))
+>this : Symbol(this, Decl(thisTypeInFunctionsNegative.ts, 174, 22))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 174, 27))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 174, 27))
+
+const f2 = <T>(this: {n: number}, m: number) => m + this.n;
+>f2 : Symbol(f2, Decl(thisTypeInFunctionsNegative.ts, 175, 5))
+>T : Symbol(T, Decl(thisTypeInFunctionsNegative.ts, 175, 12))
+>this : Symbol(this, Decl(thisTypeInFunctionsNegative.ts, 175, 15))
+>n : Symbol(n, Decl(thisTypeInFunctionsNegative.ts, 175, 22))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 175, 33))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 175, 33))
+
+const f3 = async (this: {n: number}, m: number) => m + this.n;
+>f3 : Symbol(f3, Decl(thisTypeInFunctionsNegative.ts, 176, 5))
+>this : Symbol(this, Decl(thisTypeInFunctionsNegative.ts, 176, 18))
+>n : Symbol(n, Decl(thisTypeInFunctionsNegative.ts, 176, 25))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 176, 36))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 176, 36))
+
+const f4 = async <T>(this: {n: number}, m: number) => m + this.n;
+>f4 : Symbol(f4, Decl(thisTypeInFunctionsNegative.ts, 177, 5))
+>T : Symbol(T, Decl(thisTypeInFunctionsNegative.ts, 177, 18))
+>this : Symbol(this, Decl(thisTypeInFunctionsNegative.ts, 177, 21))
+>n : Symbol(n, Decl(thisTypeInFunctionsNegative.ts, 177, 28))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 177, 39))
+>m : Symbol(m, Decl(thisTypeInFunctionsNegative.ts, 177, 39))