expr : expr '+' ident | ident

std::wstring ShiftIdent() {
    auto ident = current_token->codepoints; // for example
    current_token++;
    return ident;
}

void ShiftPlus() {
    current_token++;
}

T ShiftExpr() {
    switch(current_token->type) {
    case ident:
        ShiftIdent();
        break;
    default:
        error();
    }
    while(true) {
        switch(current_token->type) {
        case '+':
            ShiftPlus();
            auto some_var = ShiftIdent();
            break;
        default:
            auto action = [&] () -> T {
                // somestuff, which can use some_var
            };
            return action();
        };
    }
}