But that uses a cast.

Moreover, an old style cast. GNU c++ has an opt-in warning for those, -Wold-style-cast. You then need const_cast to get around that.

Then we can grep the program for that new style cast (unless it token-pasted the const_cast together in the preprocessor haha).

In C we can make such a program which contains no devices that defeat the type system, and which otherwise requires no diagnostic.