Oh, what does the variance on the left side of x and that on the right side of x correspond to in the table?
For example, V x W, Do we find V in the first element in each row? Or find V in the first element in each column?
Moreover, for a compound type, Is V the variance of the outermost type? For example:
fn(&'_ i32);
The outermost type is fn(T), in order to determine the variance of i32 in the type, we say fn(T) is contravariant over T, so V designates the variance of T? Then &'_ i32 is covariant over i32, hence W means covariant, hence the variance of i32 in the complete type is calculated by contravariant x covariant, where contravariant corresponds to the symbol in the first element of the fourth row and covariant corresponds to the symbol in the first element of the third column, the result is -, so it is contravariant?
If we continue to wrap an fn in the outside of the above type, namely, fn(fn(&'_ i32)), now the variance of i32 is the result of contravariant * contravariant, which is covariant?