Disallow yield expressions inside a class

src/compiler/checker.ts

@@ -8023,9 +8023,27 @@ module ts {
             }
         }
 
+        function isYieldExpressionInClass(node: YieldExpression): boolean {
+            let current: Node = node
+            let parent = node.parent;
+            while (parent) { 
+                if (isFunctionLike(parent) && current === (<FunctionLikeDeclaration>parent).body) {
+                    return false;
+                }
+                else if (current.kind === SyntaxKind.ClassDeclaration || current.kind === SyntaxKind.ClassExpression) {
+                    return true;
+                }
+
+                current = parent;
+                parent = parent.parent;
+            }
+
+            return false;
+        }
+
         function checkYieldExpression(node: YieldExpression): Type {
             // Grammar checking
-            if (!(node.parserContextFlags & ParserContextFlags.Yield)) {
+            if (!(node.parserContextFlags & ParserContextFlags.Yield) || isYieldExpressionInClass(node)) {
                 grammarErrorOnFirstToken(node, Diagnostics.A_yield_expression_is_only_allowed_in_a_generator_declaration);
             }
 

src/compiler/parser.ts

@@ -4010,7 +4010,20 @@ module ts {
             property.name = name;
             property.questionToken = questionToken;
             property.type = parseTypeAnnotation();
-            property.initializer = allowInAnd(parseNonParameterInitializer);
+
+            // For initializers, we always want to allow 'in' expressions. For instance properties specifically,
+            // since they are evaluated inside the constructor, we do *not* want to parse yield expressions,
+            // so we specifically turn the yield context off. The grammar would look something like this:
+            //
+            //    MemberVariableDeclaration[Yield]:
+            //        AccessibilityModifier_opt   PropertyName   TypeAnnotation_opt   Initialiser_opt[In];
+            //        AccessibilityModifier_opt  static_opt  PropertyName   TypeAnnotation_opt   Initialiser_opt[In, ?Yield];
+            //
+            // The checker may still error in the static case to explicitly disallow the yield expression.
+            property.initializer = modifiers && modifiers.flags & NodeFlags.Static
+                ? allowInAnd(parseNonParameterInitializer)
+                : doOutsideOfContext(ParserContextFlags.Yield | ParserContextFlags.DisallowIn, parseNonParameterInitializer);
+
             parseSemicolon();
             return finishNode(property);
         }

src/compiler/utilities.ts

@@ -530,11 +530,6 @@ module ts {
         return traverse(body);
 
         function traverse(node: Node): void {
-            // Yield expressions may occur in decorators
-            if (node.decorators) {
-                forEach(node.decorators, traverse);
-            }
-
             switch (node.kind) {
                 case SyntaxKind.YieldExpression:
                     visitor(<YieldExpression>node);
@@ -546,18 +541,17 @@ module ts {
                 case SyntaxKind.InterfaceDeclaration:
                 case SyntaxKind.ModuleDeclaration:
                 case SyntaxKind.TypeAliasDeclaration:
+                case SyntaxKind.ClassDeclaration:
                     // These are not allowed inside a generator now, but eventually they may be allowed
                     // as local types. Regardless, any yield statements contained within them should be
                     // skipped in this traversal.
                     return;
-                case SyntaxKind.ClassDeclaration:
-                    // A class declaration/expression may extend a yield expression
-                    forEach((<ClassDeclaration>node).heritageClauses, traverse);
-                    return;
                 default:
                     if (isFunctionLike(node)) {
                         let name = (<FunctionLikeDeclaration>node).name;
                         if (name && name.kind === SyntaxKind.ComputedPropertyName) {
+                            // Note that we will not include methods/accessors of a class because they would require
+                            // first descending into the class. This is by design.
                             traverse((<ComputedPropertyName>name).expression);
                             return;
                         }