Currently there are no traits to extract default initializers out of an aggregate type.

For example:

-----
struct S
{
    int x = 1;
    int y = 0;
    int z;
}
-----

There is a workaround: Use S.init to retrieve default values.

However, it is not reliable. Because S.init == S(1, 0, 0), but in fact 'z' does not have a *user-provided* initializer, it only has an initializer due to T.init.

This distinction is important for things like Configuration structures.

I propose a simple API: __traits(hasInitializer, ...). Then, it can be composed with fairly simply:

static if (__traits(hasInitializer, S.x))
    doSomething(S.init.x);

static if (!__traits(hasInitializer, S.z))
    doSomethingElse();