Template constraints and opAdd

[John]

I'm getting conflicting templates in this struct and I'm not sure 
how. I specifically excluded the second definition of opAdd from 
using type T in place of O but the compiler still tells me I'm 
getting template conflicts.

Compiler error using Mass!(double,string):
Error: template mass.Mass!(double,string).Mass.opAdd(O) if 
((typeof(O)) != (typeof(T))) conflicts with function 
mass.Mass!(double,string).Mass.opAdd at src\mass.d(38)


I have a struct:

struct Mass(T, S) {
        ...
        
        Mass!(T,S) opAdd(Mass!(T,S) other) {
                return op!"+"(other);
        }
        
	Mass!(O,S) opAdd(O)(Mass!(O,S) other) if (typeof(O) != 
typeof(T)) {
                return op!"+"(other);
        }
...
}

And I'm trying to do something like:

        Mass!(double,string) first = ...
        Mass!(double,string) second = ...
        
        auto result = first + second;

I'm trying to add a Mass!(double,string) + Mass!(double,string), 
which should mean the second template gets ignored since T=double 
and O=double.

What am I missing?