&gt; would be impossible to _actually_ have a feature like this as opt in since it would effectively create two different versions of C#<p>You still haven&#x27;t said what&#x27;s wrong with the proposal. There are multiple versions of C# already. Emitting warnings seems to be a good compromise, given there is already an option to treat warnings as errors.<p>&gt;`strong class Foo`<p>This is a slippery slope. Next time they add another feature for correctness, they&#x27;ll have to add &#x27;stronger class&#x27;. This is akin to &quot;use stricter&quot; proposed by Sound Script (JS).