unary minus overload: member or non-member?

Question

Given that prefix unary operators can be " implemented by a non-static member function with no parameters or a non-member function with one parameter " (§13.5.1[over.unary]/1), is there a difference besides the usual encapsulation/code reuse design rationales that apply to any member/non-member function choices?

For binary operators, there's a semantic difference because non-members allow implicit conversions of their left-hand operands. There doesn't seem to be anything like that for the unary operators, yet the standard defines std::complex 's unary negation operator as a non-member (§26.4.6[complex.ops]), while std::valarray 's and std::duration 's unary negation operators are members (§26.6.2.6[valarray.unary], §20.11.5.3[time.duration.arithmetic]). Is there a nuance?

Answer 1

As far as I'm aware there are no differences as compared to deciding if a non-operator function should be member or non-member. Obviously prefer non-member, non-friend when possible (like the standard algorithms).

Answer 2

Using members when possible makes more sense, since you don't have to go crazy with friends. But otherwise, it is just a code style decision.