This code compiles (since 2.081.2), but shouldn't: struct A { @disable this(); this(int) {} } struct B { A a; this(int) { f(a); } } void f(ref A a) {} void main() { B b = B(1); } The `a` must be constructed in the `B` constructor. But if we pass it somewhere via ref parameter, compiler ignores `a` and leaves it in default state. By the way, if we change ref to out, then there is normal `Error: cannot have out parameter of type A because the default construction is disabled`.
I think that the behavior was designed to allow for: initializeA(ref A a) { a = A(2); } struct B { A a; this(int) { initializeA(a); } } This is a common pattern and forbidding this might brake a lot of code. However, there is no way the compiler can know if the function actually initializes a or not. The problem with `out` is slightly different. Since A does not have default construction the parameter cannot be initialized before entering the function. Also, there is this issue [1], which was fixed by considering that taking the address of a variable in the constructor is the same as initializing it. Passing a field by ref to a function could be seen the same as taking its address, so by those standards this issue would be a WONTFIX. [1] https://issues.dlang.org/show_bug.cgi?id=15869