C++20 takes yet another swing at its infamous initialization rules. The players involved this time are Aggregate initialization (type a{1, 2, 3}) and direct initialization (type a(1, 2, 3)). A common pitfall with aggregate init is:

std::vector<int> vec0(5, 9); // 9, 9, 9, 9, 9
std::vector<int> vec1{5, 9}; // 5, 9

So if you don't know what you're doing, {} is potentially dangerous to use with types that might have both "real" constructors and such with std::initializer_list. If you had your head in the sand for 10 11 years and always used () then you never were in danger.