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

Fixes #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

@@ -1,4 +1,6 @@
 //// [thisTypeInFunctionsNegative.ts]
+// @target es6
+
 class C {
     n: number;
     explicitThis(this: this, m: number): number {
@@ -174,9 +176,13 @@ 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]
+// @target es6
 var __extends = (this && this.__extends) || (function () {
     var extendStatics = function (d, b) {
         extendStatics = Object.setPrototypeOf ||
@@ -190,6 +196,41 @@ var __extends = (this && this.__extends) || (function () {
         d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
     };
 })();
+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());
+    });
+};
+var __generator = (this && this.__generator) || function (thisArg, body) {
+    var _ = { label: 0, sent: function() { if (t[0] & 1) throw t[1]; return t[1]; }, trys: [], ops: [] }, f, y, t, g;
+    return g = { next: verb(0), "throw": verb(1), "return": verb(2) }, typeof Symbol === "function" && (g[Symbol.iterator] = function() { return this; }), g;
+    function verb(n) { return function (v) { return step([n, v]); }; }
+    function step(op) {
+        if (f) throw new TypeError("Generator is already executing.");
+        while (_) try {
+            if (f = 1, y && (t = op[0] & 2 ? y["return"] : op[0] ? y["throw"] || ((t = y["return"]) && t.call(y), 0) : y.next) && !(t = t.call(y, op[1])).done) return t;
+            if (y = 0, t) op = [op[0] & 2, t.value];
+            switch (op[0]) {
+                case 0: case 1: t = op; break;
+                case 4: _.label++; return { value: op[1], done: false };
+                case 5: _.label++; y = op[1]; op = [0]; continue;
+                case 7: op = _.ops.pop(); _.trys.pop(); continue;
+                default:
+                    if (!(t = _.trys, t = t.length > 0 && t[t.length - 1]) && (op[0] === 6 || op[0] === 2)) { _ = 0; continue; }
+                    if (op[0] === 3 && (!t || (op[1] > t[0] && op[1] < t[3]))) { _.label = op[1]; break; }
+                    if (op[0] === 6 && _.label < t[1]) { _.label = t[1]; t = op; break; }
+                    if (t && _.label < t[2]) { _.label = t[2]; _.ops.push(op); break; }
+                    if (t[2]) _.ops.pop();
+                    _.trys.pop(); continue;
+            }
+            op = body.call(thisArg, _);
+        } catch (e) { op = [6, e]; y = 0; } finally { f = t = 0; }
+        if (op[0] & 5) throw op[1]; return { value: op[0] ? op[1] : void 0, done: true };
+    }
+};
 var _this = this;
 var C = /** @class */ (function () {
     function C() {
@@ -360,5 +401,11 @@ number;
     return this.n;
 }
 // can't name parameters 'this' in a lambda.
-c.explicitProperty = (this, m);
-m + this.n;
+c.explicitProperty = function (m) { return m + _this.n; };
+var f2 = function (m) { return m + _this.n; };
+var f3 = function (m) { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, m + this.n];
+}); }); };
+var f4 = function (m) { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, m + this.n];
+}); }); };