Keyword to avoid not null references

[Namespace]

I thought that something like this

// not_null_struct.d

NotNull!(T) assumeNotNull(T : Object)(T t) {
	return NotNull!(T)(t);
}

@property
NotNull!(T) makeNotNull(T : Object)() {
	T t = new T();
	
	return assumeNotNull(t);
}

// not_null.d which import not_null_struct.d

NotNull!(Foo) _convert() {
	//return NotNull!(Foo)(this); // prints: Stack overflow
	return assumeNotNull(this);
}

alias _convert this;

would allow me to convert Foo to NotNull!(Foo) implicit. But it 
doesn't work. I get this error:

not_null.d(37): Error: template instance 
not_null_struct.assumeNotNull!(Foo) recursive expansion

Line 37 is the return in the _convert method.

Does anybody know why? I thought that would a smart idea...