diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 7f2b36ff7d4..7a6aaa18618 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -837,9 +837,8 @@ namespace ts {
         function isNarrowableReference(expr: Expression): boolean {
             return expr.kind === SyntaxKind.Identifier || expr.kind === SyntaxKind.ThisKeyword || expr.kind === SyntaxKind.SuperKeyword ||
                 (isPropertyAccessExpression(expr) || isNonNullExpression(expr) || isParenthesizedExpression(expr)) && isNarrowableReference(expr.expression) ||
-                isElementAccessExpression(expr) && isStringOrNumericLiteralLike(expr.argumentExpression) && isNarrowableReference(expr.expression);
-                //  ||
-                // isAssignmentExpression(expr) && isNarrowableReference(expr.left);
+                isElementAccessExpression(expr) && isStringOrNumericLiteralLike(expr.argumentExpression) && isNarrowableReference(expr.expression) ||
+                isAssignmentExpression(expr) && isNarrowableReference(expr.left);
         }
 
         function containsNarrowableReference(expr: Expression): boolean {
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d386adc352d..badc8a5ed9b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -20131,11 +20131,11 @@ namespace ts {
                 case SyntaxKind.ParenthesizedExpression:
                 case SyntaxKind.NonNullExpression:
                     return isMatchingReference(source, (target as NonNullExpression | ParenthesizedExpression).expression);
-                // case SyntaxKind.BinaryExpression:
-                //     if (isAssignmentExpression(target)) {
-                //         return isMatchingReference(source, target.left);
-                //     }
-                //     break;
+                case SyntaxKind.BinaryExpression:
+                    if (isAssignmentExpression(target)) {
+                        return isMatchingReference(source, target.left);
+                    }
+                    break;
             }
             switch (source.kind) {
                 case SyntaxKind.Identifier: