Cast-expression that casts to the type void
可以利用“the argument of a cast-expression that casts to the type void is a discard-value expression”来实现一些效果，下面是一些案例：
1、cppreference Expression SFINAE on the return type
A common idiom is to use expression SFINAE on the return type, where the expression uses the comma operator, whose left subexpression is the one that is being examined (cast to
voidto ensure the user-defined operator comma on the returned type is not selected), and the right subexpression has the type that the function is supposed to return.
2、stackoverflow How do I best silence a warning about unused variables? # A
// silence a warning about unused variables，https://stackoverflow.com/a/1486931
#define UNUSED(expr) do { (void)(expr); } while (0)
Example: 在cppreference static_cast conversion的4)就是说明的discard value expression，Example也对此进行了说明。