How get struct value by member name string ?

[John Xu via Digitalmars-d-learn]

I saw ddbc 
(https://github.com/buggins/ddbc/blob/master/source/ddbc/pods.d) 
uses

    static if (__traits(compiles, (typeof(__traits(getMember, T, 
m))))) {
        __traits(getMember, T, m)
    }
But for my experience, above code sometimes/somewhere works, 
sometimes/somewhere just doesn't:

    Error: variable `column` cannot be read at compile time

Is there any friend can explain some more details?