Some assignments should be able to execute in safe code

[iK4tsu]

void main() @safe
{
	alias SM = SumType!string;
	auto sm = SM("hummm");
	sm = SM("this should be @safe");
}

[pbackus]

I think the best that can be done here is to allow the assignment if the type being assigned is the only member type that has unsafe values. This would make the example above @safe, but would still require assignments to a SumType!(string, int[]) to be @system.