D-Programming-Language/dmd — comment on commit 675898721c04d0bf155a85abf986eae99c37c0dc
File:  line 0
By simendsjo:

That works for structs, but for class I get "Error: no constructor for C".
Even if it did work, rearranging the fields, or adding new ones, would give unexpected results. It's a feature I don't dare to use.. Adding a constructor that has the same number and types of parameters, but that doesn't assign in the same order would also compile without a warning, but could change the code completely.

auto c = new C(); with(c) {
a = 1;
b = 2;
}

Is a safer trick, although a bit longer to write.