Use union type when binding element has initializer

src/compiler/checker.ts

@@ -2974,7 +2974,9 @@ namespace ts {
             if (strictNullChecks && declaration.initializer && !(getFalsyFlags(checkExpressionCached(declaration.initializer)) & TypeFlags.Undefined)) {
                 type = getTypeWithFacts(type, TypeFacts.NEUndefined);
             }
-            return type;
+            return declaration.initializer ?
+                getUnionType([type, checkExpressionCached(declaration.initializer)], /*subtypeReduction*/ true) :
+                type;
         }
 
         function getTypeForVariableLikeDeclarationFromJSDocComment(declaration: VariableLikeDeclaration) {