Ok, that is useful to know. In this case, the constructor is also doing the extra work of doing the interpolation. The thing that confuses me is that if I do @code_warntype m.itp(x), that is fine, but f(m,x) is not even when the type information is present in m; I think this is quite similar to the question from Type instability if function inside a struct is run inside another function , and my original solution was an attempt to emulate the accepted solution from that question