Sparse Aggregate Assignment/Initialization (RAII)

["Nordlöw" via Digitalmars-d-learn]

Because D currently doesn't support RAII using named parameters 
like in Python I tried the following.

Say I have an aggregate

class C { int x,y,z,w; }

or similarly

struct C { int x,y,z,w; }

I know define a generic _free_ function

ref T set(string member, T, U)(ref T a, in U value) if 
(isAggregateType!T &&
                                                         
hasMember!(T, member))
{
     mixin(`a.` ~ member ~ ` = value;`);
     return a;
}

which I want to use for flexible initialization of several 
members at once.

However using this function through UFCS

     auto cx = new C().set!"x"(11);

fails as

algorithm_ex.d(1257,17): Error: template algorithm_ex.set cannot 
deduce function from argument types !("x")(C, int), candidates 
are:
algorithm_ex.d(1242,7):        algorithm_ex.set(string member, T, 
U)(ref T a, in U value) if (isAggregateType!T && hasMember!(T, 
member))

Instead I have to use

     auto c = new C(); set!"x"(c, 11);

which is not as elegant.

Why doesn't UCFS work here and is there a solution using DMD git 
master?

Further Is there a cleverer way to do this without resorting to 
mixins?

This old post

http://forum.dlang.org/thread/mailman.2966.1301533296.4748.digitalmars-d-learn@puremagic.com

talks about opDispatch() but, to my knowledge, it requires the 
aggregate to have extra members doing the initialization.

I want this because I've identified in some parts of my code that 
I have large structures were only a few of the elements are 
initialized to non-default values.