Improved lookahead for arrow functions.

src/compiler/parser.ts

@@ -1445,64 +1445,87 @@ module ts {
         function tryParseParenthesizedArrowFunctionExpression(): Expression {
             var pos = getNodePos();
 
+            // Indicates whether we are certain that we should parse an arrow expression.
             var triState = isParenthesizedArrowFunctionExpression();
 
-            // It is not a parenthesized arrow function.
             if (triState === Tristate.False) {
                 return undefined;
             }
 
-            // If we're certain that we have an arrow function expression, then just parse one out.
             if (triState === Tristate.True) {
                 var sig = parseSignature(SyntaxKind.CallSignature, SyntaxKind.ColonToken);
 
                 // If we have an arrow, then try to parse the body.
-                if (parseExpected(SyntaxKind.EqualsGreaterThanToken)) {
-                    return parseArrowExpressionTail(pos, sig, /*noIn:*/ false);
+                // Even if not, try to parse if we have an opening brace, just in case we're in an error state.
+                if (parseExpected(SyntaxKind.EqualsGreaterThanToken) || token === SyntaxKind.OpenBraceToken) {
+                    return parseArrowExpressionTail(pos, sig, /* noIn: */ false);
                 }
-                // If not, we're probably better off bailing out and returning a bogus function expression.
                 else {
+                    // If not, we're probably better off bailing out and returning a bogus function expression.
                     return makeFunctionExpression(SyntaxKind.ArrowFunction, pos, /* name */ undefined, sig, createMissingNode());
                 }
             }
 
-            // Otherwise, *maybe* we had an arrow function and we need to *try* to parse it out
-            // (which will ensure we rollback if we fail).
-            var sig = tryParse(parseSignatureAndArrow);
+            // *Maybe* we had an arrow function and we need to try to parse it out,
+            // rolling back and trying other parses if we fail.
+            var sig = tryParse(parseSignatureIfArrowOrBraceFollows);
             if (sig === undefined) {
                 return undefined;
             }
             else {
+                parseExpected(SyntaxKind.EqualsGreaterThanToken);
                 return parseArrowExpressionTail(pos, sig, /*noIn:*/ false);
             }
         }
 
-        //  True        -> There is definitely a parenthesized arrow function here. 
-        //  False       -> There is definitely *not* a parenthesized arrow function here.
+        //  True        -> We definitely expect a parenthesized arrow function here.
+        //  False       -> There *cannot* be a parenthesized arrow function here.
         //  Unknown     -> There *might* be a parenthesized arrow function here.
-        //                  Speculatively look ahead to be sure.
+        //                 Speculatively look ahead to be sure, and rollback if not.
         function isParenthesizedArrowFunctionExpression(): Tristate {
             if (token === SyntaxKind.OpenParenToken || token === SyntaxKind.LessThanToken) {
                 return lookAhead(() => {
                     var first = token;
-                    nextToken();
+                    var second = nextToken();
+
                     if (first === SyntaxKind.OpenParenToken) {
-                        if (token === SyntaxKind.CloseParenToken || token === SyntaxKind.DotDotDotToken) {
-                            // Simple cases.  if we see ()   or   (...   then presume that presume 
-                            // that this must be an arrow function.  Note, this may be too aggressive
-                            // for the "()" case.  It's not uncommon for this to appear while editing 
-                            // code.  We should look to see if there's actually a => before proceeding.
+                        if (second === SyntaxKind.CloseParenToken) {
+                            // Simple cases: "() =>", "(): ", and  "() {".
+                            // This is an arrow function with no parameters.
+                            // The last one is not actually an arrow function,
+                            // but this is probably what the user intended.
+                            var third = nextToken();
+                            switch (third) {
+                                case SyntaxKind.EqualsGreaterThanToken:
+                                case SyntaxKind.ColonToken:
+                                case SyntaxKind.OpenBraceToken:
+                                    return Tristate.True;
+                                default:
+                                    return Tristate.False;
+                            }
+                        }
+
+                        // Simple case: "(..."
+                        // This is an arrow function with a rest parameter.
+                        if (second === SyntaxKind.DotDotDotToken) {
                             return Tristate.True;
                         }
 
+                        // If we had "(" followed by something that's not an identifier,
+                        // then this definitely doesn't look like a lambda.
+                        // Note: we could be a little more lenient and allow
+                        // "(public" or "(private". These would not ever actually be allowed,
+                        // but we could provide a good error message instead of bailing out.
                         if (!isIdentifier()) {
-                            // We had "(" not followed by an identifier.  This definitely doesn't 
-                            // look like a lambda.  Note: we could be a little more lenient and allow
-                            // (public   or  (private.  These would not ever actually be allowed,
-                            // but we could provide a good error message instead of bailing out.
                             return Tristate.False;
                         }
 
+                        // If we have something like "(a:", then we must have a
+                        // type-annotated parameter in an arrow function expression.
+                        if (nextToken() === SyntaxKind.ColonToken) {
+                            return Tristate.True;
+                        }
+
                         // This *could* be a parenthesized arrow function.
                         // Return Unknown to let the caller know.
                         return Tristate.Unknown;
@@ -1526,10 +1549,22 @@ module ts {
             return Tristate.False;
         }
 
-        function parseSignatureAndArrow(): ParsedSignature {
+        function parseSignatureIfArrowOrBraceFollows(): ParsedSignature {
             var sig = parseSignature(SyntaxKind.CallSignature, SyntaxKind.ColonToken);
-            parseExpected(SyntaxKind.EqualsGreaterThanToken);
-            return sig;
+
+            // Parsing a signature isn't enough.
+            // Parenthesized arrow signatures often look like other valid expressions.
+            // For instance:
+            //  - "(x = 10)" is an assignment expression parsed as a signature with a default parameter value.
+            //  - "(x,y)" is a comma expression parsed as a signature with two parameters.
+            //  - "a ? (b): c" will have "(b):" parsed as a signature with a return type annotation.
+            //
+            // So we need just a bit of lookahead to ensure that it can only be a signature.
+            if (token === SyntaxKind.EqualsGreaterThanToken || token === SyntaxKind.OpenBraceToken) {
+                return sig;
+            }
+
+            return undefined;
         }
 
         function parseArrowExpressionTail(pos: number, sig: ParsedSignature, noIn: boolean): FunctionExpression {

tests/baselines/reference/arrowFunctionsMissingTokens.errors.txt

@@ -0,0 +1,115 @@
+==== tests/cases/compiler/arrowFunctionsMissingTokens.ts (24 errors) ====
+
+    module missingArrowsWithCurly {
+        var a = () { };
+                   ~
+!!! '=>' expected.
+
+        var b = (): void { }
+                         ~
+!!! '=>' expected.
+
+        var c = (x) { };
+                    ~
+!!! '=>' expected.
+
+        var d = (x: number, y: string) { };
+                                       ~
+!!! '=>' expected.
+
+        var e = (x: number, y: string): void { };
+                                             ~
+!!! '=>' expected.
+    }
+
+    module missingCurliesWithArrow {
+        module withStatement {
+            var a = () => var k = 10;};
+                          ~~~
+!!! '{' expected.
+
+            var b = (): void => var k = 10;}
+                                ~~~
+!!! '{' expected.
+
+            var c = (x) => var k = 10;};
+                           ~~~
+!!! '{' expected.
+
+            var d = (x: number, y: string) => var k = 10;};
+                                              ~~~
+!!! '{' expected.
+
+            var e = (x: number, y: string): void => var k = 10;};
+                                                    ~~~
+!!! '{' expected.
+
+            var f = () => var k = 10;}
+                          ~~~
+!!! '{' expected.
+        }
+
+        module withoutStatement {
+            var a = () => };
+                          ~
+!!! Expression expected.
+
+            var b = (): void => }
+                                ~
+!!! Expression expected.
+
+            var c = (x) => };
+                           ~
+!!! Expression expected.
+
+            var d = (x: number, y: string) => };
+                                              ~
+!!! Expression expected.
+
+            var e = (x: number, y: string): void => };
+                                                    ~
+!!! Expression expected.
+
+            var f = () => }
+                          ~
+!!! Expression expected.
+        }
+        ~
+!!! Declaration or statement expected.
+    }
+    ~
+!!! Declaration or statement expected.
+
+    module ceci_nEst_pas_une_arrow_function {
+        var a = ();
+                 ~
+!!! Expression expected.
+
+        var b = (): void;
+                        ~
+!!! '=>' expected.
+
+        var c = (x);
+                 ~
+!!! Cannot find name 'x'.
+
+        var d = (x: number, y: string);
+                                      ~
+!!! '=>' expected.
+
+        var e = (x: number, y: string): void;
+                                            ~
+!!! '=>' expected.
+    }
+
+    module okay {
+        var a = () => { };
+
+        var b = (): void => { }
+
+        var c = (x) => { };
+
+        var d = (x: number, y: string) => { };
+
+        var e = (x: number, y: string): void => { };
+    }

tests/baselines/reference/emptyMemberAccess.errors.txt

@@ -1,11 +1,9 @@
-==== tests/cases/compiler/emptyMemberAccess.ts (2 errors) ====
+==== tests/cases/compiler/emptyMemberAccess.ts (1 errors) ====
     function getObj() {
 
        ().toString();
-         ~
-!!! '=>' expected.
-          ~~~~~~~~
-!!! Cannot find name 'toString'.
+        ~
+!!! Expression expected.
 
     }
 

tests/baselines/reference/es6ClassTest9.errors.txt

@@ -2,8 +2,8 @@
     declare class foo();
                      ~
 !!! '{' expected.
-                       ~
-!!! '=>' expected.
+                      ~
+!!! Expression expected.
     function foo() {}
              ~~~
 !!! Duplicate identifier 'foo'.

tests/baselines/reference/fatarrowfunctionsErrors.errors.txt

@@ -1,4 +1,4 @@
-==== tests/cases/compiler/fatarrowfunctionsErrors.ts (25 errors) ====
+==== tests/cases/compiler/fatarrowfunctionsErrors.ts (18 errors) ====
     foo((...Far:any[])=>{return 0;})
     ~~~
 !!! Cannot find name 'foo'.
@@ -39,25 +39,11 @@
                       ~
 !!! '=>' expected.
     var x2 = (a:number) :void {};
-               ~
-!!! ')' expected.
-                      ~
-!!! ',' expected.
-                        ~
-!!! Variable declaration expected.
-                         ~~~~
-!!! Variable declaration expected.
-              ~
-!!! Cannot find name 'a'.
+                              ~
+!!! '=>' expected.
     var x3 = (a:number) {};
-               ~
-!!! ')' expected.
-                      ~
-!!! ',' expected.
                         ~
-!!! Variable declaration expected.
-              ~
-!!! Cannot find name 'a'.
+!!! '=>' expected.
     var x4= (...a: any[]) { };
                           ~
 !!! '=>' expected.

tests/baselines/reference/invalidTryStatements2.errors.txt

@@ -1,4 +1,4 @@
-==== tests/cases/conformance/statements/tryStatements/invalidTryStatements2.ts (5 errors) ====
+==== tests/cases/conformance/statements/tryStatements/invalidTryStatements2.ts (4 errors) ====
     function fn() {
         try {
         } catch { // syntax error, missing '(x)'
@@ -10,9 +10,7 @@
         ~~~~~
 !!! Statement expected.
                  ~
-!!! ';' expected.
-              ~
-!!! Cannot find name 'x'.
+!!! '=>' expected.
 
         finally{ } // error missing try
         ~~~~~~~

tests/baselines/reference/parser566700.errors.txt

@@ -1,4 +1,4 @@
 ==== tests/cases/conformance/parser/ecmascript5/RegressionTests/parser566700.ts (1 errors) ====
     var v = ()({});
-              ~
-!!! '=>' expected.
+             ~
+!!! Expression expected.

tests/baselines/reference/parserEmptyParenthesizedExpression1.errors.txt

@@ -1,8 +1,6 @@
-==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/parserEmptyParenthesizedExpression1.ts (2 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/parserEmptyParenthesizedExpression1.ts (1 errors) ====
     function getObj() {
        ().toString();
-         ~
-!!! '=>' expected.
-          ~~~~~~~~
-!!! Cannot find name 'toString'.
+        ~
+!!! Expression expected.
     }

tests/baselines/reference/parserErrorRecovery_Expression1.errors.txt

@@ -1,4 +1,4 @@
 ==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/Expressions/parserErrorRecovery_Expression1.ts (1 errors) ====
     var v = ()({});
-              ~
-!!! '=>' expected.
+             ~
+!!! Expression expected.

tests/baselines/reference/parserErrorRecovery_ParameterList5.errors.txt

@@ -1,10 +1,6 @@
-==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/ParameterLists/parserErrorRecovery_ParameterList5.ts (4 errors) ====
+==== tests/cases/conformance/parser/ecmascript5/ErrorRecovery/ParameterLists/parserErrorRecovery_ParameterList5.ts (2 errors) ====
     (a:number => { }
-      ~
-!!! ')' expected.
               ~~
-!!! ';' expected.
-     ~
-!!! Cannot find name 'a'.
-       ~~~~~~
-!!! Cannot find name 'number'.
+!!! ',' expected.
+                 ~
+!!! ')' expected.