Is the value category of a fold expression always prvalue?

Question

Is a fold expression always a prvalue? Is this correct?

template<typename... Args>
auto sum(Args... args) {
    auto fold = (... +  args);
    return fold;
}

int main() {
    sum(10, 2, 2);
}

I‘m really only interested in the fold expression which is (... + args) in the above example.

Answer 1

A fold-expression has the same semantics as simply writing out the N - 1 applications of the operator (where N is the number of elements in the pack). For example, sum(10, 2, 2) will produce (10 + 2) + 2. See [temp.variadic]/9.

In general, this may or may not be an prvalue. Folding 2 or more numeric values with + will always yield a prvalue since the built-in + operator yields a prvalue, but if there is only one element in the pack args, then (... + args) means the same thing as simply mentioning that one element by its hypothetical name, so the result would be an lvalue. And of course you can fold with other (possibly overloaded) operators too, which may produce glvalues.