Const.

src/compiler/checker.ts

@@ -10403,7 +10403,7 @@ namespace ts {
         function checkStringLiteralExpression(node: StringLiteral): Type {
             // TODO (drosen): Do we want to apply the same approach to no-sub template literals?
 
-            let contextualType = getContextualType(node);
+            const contextualType = getContextualType(node);
             if (contextualType && contextualTypeIsStringLiteralType(contextualType)) {
                 return getStringLiteralType(node);
             }