Parse type predicates only in return types.

src/compiler/parser.ts

@@ -908,17 +908,19 @@ namespace ts {
             return result;
         }
 
-        // Invokes the provided callback then unconditionally restores the parser to the state it
-        // was in immediately prior to invoking the callback.  The result of invoking the callback
-        // is returned from this function.
+        /** Invokes the provided callback then unconditionally restores the parser to the state it
+         * was in immediately prior to invoking the callback.  The result of invoking the callback
+         * is returned from this function. 
+         */
         function lookAhead<T>(callback: () => T): T {
             return speculationHelper(callback, /*isLookAhead*/ true);
         }
 
-        // Invokes the provided callback.  If the callback returns something falsy, then it restores
-        // the parser to the state it was in immediately prior to invoking the callback.  If the
-        // callback returns something truthy, then the parser state is not rolled back.  The result
-        // of invoking the callback is returned from this function.
+        /** Invokes the provided callback.  If the callback returns something falsy, then it restores
+         * the parser to the state it was in immediately prior to invoking the callback.  If the
+         * callback returns something truthy, then the parser state is not rolled back.  The result
+         * of invoking the callback is returned from this function.
+         */
         function tryParse<T>(callback: () => T): T {
             return speculationHelper(callback, /*isLookAhead*/ false);
         }
@@ -1960,15 +1962,8 @@ namespace ts {
 
         // TYPES
 
-        function parseTypeReferenceOrTypePredicate(): TypeReferenceNode | TypePredicateNode {
+        function parseTypeReference(): TypeReferenceNode {
             const typeName = parseEntityName(/*allowReservedWords*/ false, Diagnostics.Type_expected);
-            if (typeName.kind === SyntaxKind.Identifier && token === SyntaxKind.IsKeyword && !scanner.hasPrecedingLineBreak()) {
-                nextToken();
-                const node = <TypePredicateNode>createNode(SyntaxKind.TypePredicate, typeName.pos);
-                node.parameterName = <Identifier>typeName;
-                node.type = parseType();
-                return finishNode(node);
-            }
             const node = <TypeReferenceNode>createNode(SyntaxKind.TypeReference, typeName.pos);
             node.typeName = typeName;
             if (!scanner.hasPrecedingLineBreak() && token === SyntaxKind.LessThanToken) {
@@ -2100,10 +2095,10 @@ namespace ts {
 
             if (returnTokenRequired) {
                 parseExpected(returnToken);
-                signature.type = parseType();
+                signature.type = parseTypeOrTypePredicate();
             }
             else if (parseOptional(returnToken)) {
-                signature.type = parseType();
+                signature.type = parseTypeOrTypePredicate();
             }
         }
 
@@ -2419,7 +2414,7 @@ namespace ts {
                 case SyntaxKind.SymbolKeyword:
                     // If these are followed by a dot, then parse these out as a dotted type reference instead.
                     const node = tryParse(parseKeywordAndNoDot);
-                    return node || parseTypeReferenceOrTypePredicate();
+                    return node || parseTypeReference();
                 case SyntaxKind.StringLiteral:
                     return parseStringLiteralTypeNode();
                 case SyntaxKind.VoidKeyword:
@@ -2435,7 +2430,7 @@ namespace ts {
                 case SyntaxKind.OpenParenToken:
                     return parseParenthesizedType();
                 default:
-                    return parseTypeReferenceOrTypePredicate();
+                    return parseTypeReference();
             }
         }
 
@@ -2541,6 +2536,26 @@ namespace ts {
             }
             return false;
         }
+       
+        function parseTypeOrTypePredicate(): TypeNode {
+            const typePredicateVariable = tryParse(() => {
+                const id = parseIdentifier();
+                if (token === SyntaxKind.IsKeyword && !scanner.hasPrecedingLineBreak()) {
+                    nextToken();
+                    return id;
+                }
+            });
+            const t = parseType();
+            if(typePredicateVariable) {
+                const node = <TypePredicateNode>createNode(SyntaxKind.TypePredicate, typePredicateVariable.pos);
+                node.parameterName = typePredicateVariable;
+                node.type = t;
+                return finishNode(node);
+            }
+            else {
+                return t;
+            }
+        }
 
         function parseType(): TypeNode {
             // The rules about 'yield' only apply to actual code/expression contexts.  They don't