我有一个代码在上下文中使用带有参数包的概念if constexpr
。它曾经在 gcc 8 中编译和工作,但在 gcc 9 中中断:
#include <utility>
#include <tuple>
template <typename T, typename ...Ts>
concept
#if __GNUC__ < 9
bool
#endif
is_handled = requires(T handler, Ts && ...args) {
handler.handle(std::forward<Ts>(args)...);
};
template <typename T>
concept
#if __GNUC__ < 9
bool
#endif
has_name = requires(T) {
T::name;
};
template <typename ...Handlers>
struct Dispatcher {
template <typename ...Ts>
void operator()(Ts && ...args) {
std::apply(
[&](auto && ...handlers) {
(handle(handlers, std::forward<Ts>(args)...),...);
},
m_handlers
);
}
private:
template <typename Handler, typename ...Ts>
void handle(Handler & handler, Ts && ...args) {
if constexpr (is_handled<Handler, Ts...>) {
handler.handle(std::forward<Ts>(args)...);
}
}
template <typename Handler>
char const* get_code() {
if constexpr (has_name<Handler>) {
return Handler::name;
}
return nullptr;
}
std::tuple<Handlers...> m_handlers;
};
GCC 9.2 产生的错误是:
<source>: In member function 'void Dispatcher<Handlers>::handle(Handler&, Ts&& ...)':
<source>:38:49: error: expected unqualified-id before ')' token
38 | if constexpr (is_handled<Handler, Ts...>) {
| ^
请参阅此处的编译器资源管理器。您可以在 gcc 版本 8.3 和 9.2 之间切换以查看差异。