fix parse error for arrow function in rhs

src/compiler/diagnosticMessages.json

@@ -4485,6 +4485,10 @@
         "category": "Error",
         "code": 17017
     },
+    "Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.": {
+        "category": "Error",
+        "code": 17018
+    },
 
     "Circularity detected while resolving configuration: {0}": {
         "category": "Error",

src/compiler/parser.ts

@@ -3857,13 +3857,47 @@ namespace ts {
                     }
                 }
                 else {
-                    leftOperand = makeBinaryExpression(leftOperand, parseTokenNode(), parseBinaryExpressionOrHigher(newPrecedence));
+                    leftOperand = makeBinaryExpression(leftOperand, parseTokenNode<Token<BinaryOperator>>(), tryParseArrowFunctionExpressionOrExpression() || parseBinaryExpressionOrHigher(newPrecedence));
                 }
             }
-
             return leftOperand;
         }
 
+        function tryParseArrowFunctionExpressionOrExpression() {
+            // Much of the time, users will write code along the lines of
+            //
+            //   let x = foo || () => { /*...*/ }
+            //
+            // However, arrow functions aren't valid in those positions in the ECMAScript grammar.
+            // Despite that, we'll try to parse it out anyway and give a decent error message.
+            if (isParenthesizedArrowFunctionExpression() !== Tristate.False) {
+                const maybeArrowFunction = lookAhead(tryParseParenthesizedArrowFunctionExpression);
+                if (maybeArrowFunction) {
+                    parseErrorAtNode(maybeArrowFunction, Diagnostics.Arrow_functions_are_not_syntactically_valid_here_Consider_wrapping_the_function_in_parentheses);
+                    return maybeArrowFunction;
+                }
+            }
+            if (lookAhead(isStartOfSimpleArrowFunction)) {
+                const arrowFunction = tryParseAsyncSimpleArrowFunctionExpression() || parseSimpleArrowFunctionExpression(<Identifier>parseBinaryExpressionOrHigher(/*precedence*/ 0));
+                parseErrorAtNode(arrowFunction, Diagnostics.Arrow_functions_are_not_syntactically_valid_here_Consider_wrapping_the_function_in_parentheses);
+                return arrowFunction;
+            }
+            return undefined;
+        }
+
+        function parseErrorAtNode(node: Node, message: DiagnosticMessage, arg0?: any): void {
+            const start = skipTrivia(sourceFile.text, node.pos)
+            parseErrorAtPosition(start, node.end - start, message, arg0)
+        }
+
+        function isStartOfSimpleArrowFunction() {
+            if (token() === SyntaxKind.AsyncKeyword) {
+                nextToken();
+                return isIdentifier() && nextToken() === SyntaxKind.EqualsGreaterThanToken || token() === SyntaxKind.EqualsGreaterThanToken;
+            }
+            return isIdentifier() && nextToken() === SyntaxKind.EqualsGreaterThanToken;
+        }
+
         function isBinaryOperator() {
             if (inDisallowInContext() && token() === SyntaxKind.InKeyword) {
                 return false;

tests/baselines/reference/ambiguousGenericAssertion1.errors.txt

@@ -1,23 +1,26 @@
 tests/cases/compiler/ambiguousGenericAssertion1.ts(4,10): error TS1109: Expression expected.
-tests/cases/compiler/ambiguousGenericAssertion1.ts(4,15): error TS2304: Cannot find name 'x'.
-tests/cases/compiler/ambiguousGenericAssertion1.ts(4,16): error TS1005: ')' expected.
-tests/cases/compiler/ambiguousGenericAssertion1.ts(4,19): error TS1005: ',' expected.
-tests/cases/compiler/ambiguousGenericAssertion1.ts(4,21): error TS1005: ';' expected.
+tests/cases/compiler/ambiguousGenericAssertion1.ts(4,10): error TS2365: Operator '>' cannot be applied to types 'number' and '(x: any) => boolean'.
+tests/cases/compiler/ambiguousGenericAssertion1.ts(4,12): error TS2304: Cannot find name 'T'.
+tests/cases/compiler/ambiguousGenericAssertion1.ts(4,14): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/ambiguousGenericAssertion1.ts(4,18): error TS2304: Cannot find name 'T'.
+tests/cases/compiler/ambiguousGenericAssertion1.ts(4,24): error TS2304: Cannot find name 'T'.
 
 
-==== tests/cases/compiler/ambiguousGenericAssertion1.ts (5 errors) ====
+==== tests/cases/compiler/ambiguousGenericAssertion1.ts (6 errors) ====
     function f<T>(x: T): T { return null; }
     var r = <T>(x: T) => x;
     var r2 = < <T>(x: T) => T>f; // valid
     var r3 = <<T>(x: T) => T>f; // ambiguous, appears to the parser as a << operation
              ~~
 !!! error TS1109: Expression expected.
-                  ~
-!!! error TS2304: Cannot find name 'x'.
-                   ~
-!!! error TS1005: ')' expected.
-                      ~
-!!! error TS1005: ',' expected.
-                        ~~
-!!! error TS1005: ';' expected.
+             ~~~~
+!!! error TS2365: Operator '>' cannot be applied to types 'number' and '(x: any) => boolean'.
+               ~
+!!! error TS2304: Cannot find name 'T'.
+                 ~~~~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+                     ~
+!!! error TS2304: Cannot find name 'T'.
+                           ~
+!!! error TS2304: Cannot find name 'T'.
 

tests/baselines/reference/ambiguousGenericAssertion1.js

@@ -9,5 +9,5 @@ var r3 = <<T>(x: T) => T>f; // ambiguous, appears to the parser as a << operatio
 function f(x) { return null; }
 var r = function (x) { return x; };
 var r2 = f; // valid
-var r3 =  << T > (x), T;
-T > f; // ambiguous, appears to the parser as a << operation
+var r3 =  << T > (function (x) { return T > f; });
+(function (x) { return T > f; }); // ambiguous, appears to the parser as a << operation

tests/baselines/reference/ambiguousGenericAssertion1.symbols

@@ -23,8 +23,8 @@ var r2 = < <T>(x: T) => T>f; // valid
 
 var r3 = <<T>(x: T) => T>f; // ambiguous, appears to the parser as a << operation
 >r3 : Symbol(r3, Decl(ambiguousGenericAssertion1.ts, 3, 3))
->T : Symbol(T, Decl(ambiguousGenericAssertion1.ts, 3, 16))
->T : Symbol(T, Decl(ambiguousGenericAssertion1.ts, 3, 16))
->T : Symbol(T, Decl(ambiguousGenericAssertion1.ts, 3, 16))
+>x : Symbol(x, Decl(ambiguousGenericAssertion1.ts, 3, 14))
+>f : Symbol(f, Decl(ambiguousGenericAssertion1.ts, 0, 0))
+>x : Symbol(x, Decl(ambiguousGenericAssertion1.ts, 3, 14))
 >f : Symbol(f, Decl(ambiguousGenericAssertion1.ts, 0, 0))
 

tests/baselines/reference/ambiguousGenericAssertion1.types

@@ -18,13 +18,17 @@ var r2 = < <T>(x: T) => T>f; // valid
 
 var r3 = <<T>(x: T) => T>f; // ambiguous, appears to the parser as a << operation
 >r3 : boolean
-><<T>(x : boolean
+><<T> : boolean
 ><<T : number
 > : any
 >T : any
->(x : any
+>(x: T) => T>f : (x: any) => boolean
 >x : any
+>T>f : boolean
 >T : any
+>f : <T>(x: T) => T
+>(x: T) => T>f : (x: any) => boolean
+>x : any
 >T>f : boolean
 >T : any
 >f : <T>(x: T) => T

tests/baselines/reference/incorrectArrowFunction.errors.txt

@@ -0,0 +1,65 @@
+tests/cases/compiler/incorrectArrowFunction.ts(7,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(8,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(9,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(10,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(11,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(12,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(13,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(14,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(15,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+tests/cases/compiler/incorrectArrowFunction.ts(16,9): error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+
+
+==== tests/cases/compiler/incorrectArrowFunction.ts (10 errors) ====
+    declare const foo;
+
+    interface Foo {
+        a: number
+    }
+
+    true || () => 1;
+            ~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || a => 1;
+            ~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || a => b => 1;
+            ~~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || (a) => 2;
+            ~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || foo => 1;
+            ~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || (a: number) => 1;
+            ~~~~~~~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || ({ a }: Foo) => 1;
+            ~~~~~~~~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || async() => 1;
+            ~~~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || async a => 1;
+            ~~~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+    true || async => 1;
+            ~~~~~~~~~~
+!!! error TS17018: Arrow functions are not syntactically valid here. Consider wrapping the function in parentheses.
+
+    true || (() => 1);
+    true || (a => 1);
+    true || ((a) => 2);
+    true || (foo => 1);
+    true || ((a: number) => 1);
+    true || (({ a }: Foo) => 1);
+    true || (async () => 1);
+    true || (async a => 1);
+    true || (async => 1);
+    (() => 1) || (() => 2);
+
+    true || (false);
+    true || false;
+    true || foo;
+

tests/baselines/reference/incorrectArrowFunction.js

@@ -0,0 +1,118 @@
+//// [incorrectArrowFunction.ts]
+declare const foo;
+
+interface Foo {
+    a: number
+}
+
+true || () => 1;
+true || a => 1;
+true || a => b => 1;
+true || (a) => 2;
+true || foo => 1;
+true || (a: number) => 1;
+true || ({ a }: Foo) => 1;
+true || async() => 1;
+true || async a => 1;
+true || async => 1;
+
+true || (() => 1);
+true || (a => 1);
+true || ((a) => 2);
+true || (foo => 1);
+true || ((a: number) => 1);
+true || (({ a }: Foo) => 1);
+true || (async () => 1);
+true || (async a => 1);
+true || (async => 1);
+(() => 1) || (() => 2);
+
+true || (false);
+true || false;
+true || foo;
+
+
+//// [incorrectArrowFunction.js]
+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;
+true || function () { return 1; };
+(function () { return 1; });
+true || function (a) { return 1; };
+true || function (a) { return function (b) { return 1; }; };
+true || function (a) { return 2; };
+(function (a) { return 2; });
+true || function (foo) { return 1; };
+true || (function (a) { return 1; });
+(function (a) { return 1; });
+true || (function (_a) {
+    var a = _a.a;
+    return 1;
+});
+(function (_a) {
+    var a = _a.a;
+    return 1;
+});
+true || (function () { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, 1];
+}); }); });
+(function () { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, 1];
+}); }); });
+true || (function (a) { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, 1];
+}); }); });
+true || function (async) { return 1; };
+true || (function () { return 1; });
+true || (function (a) { return 1; });
+true || (function (a) { return 2; });
+true || (function (foo) { return 1; });
+true || (function (a) { return 1; });
+true || (function (_a) {
+    var a = _a.a;
+    return 1;
+});
+true || (function () { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, 1];
+}); }); });
+true || (function (a) { return __awaiter(_this, void 0, void 0, function () { return __generator(this, function (_a) {
+    return [2 /*return*/, 1];
+}); }); });
+true || (function (async) { return 1; });
+(function () { return 1; }) || (function () { return 2; });
+true || (false);
+true || false;
+true || foo;