问题描述
以下代码要求在 gcc 中的std::decay
中使用noexcept operator
,但不能在 clang 中使用。
template<typename... Ts>
class B;
template<typename T>
class B<T> {
T t;
public:
template<typename U>
constexpr B(U&& t)
// without decay - strange behavior in gcc,see main below <===
noexcept(noexcept(T{std::forward<U>(t)}))
// if adding decay - all cases in main are ok with both gcc and clang
// noexcept(noexcept(std::decay_t<T>{std::forward<U>(t)}))
: t(std::forward<U>(t)) {}
};
template<typename T,typename... Ts>
class B<T,Ts...>: protected B<Ts...> {
public:
template<typename U,typename... Us>
constexpr B(U&& t,Us&&... ts)
: B<Ts...>{std::forward<Us>(ts)...} {}
};
template<typename... Ts>
constexpr auto create(Ts&&... ts) {
return B<Ts...>{std::forward<Ts>(ts)...};
}
template<typename... Ts>
B(Ts...) -> B<Ts...>;
主要
int main() {
// ok in both gcc and clang:
// [1] the "hello" parameter is not last
auto b1 = create("hello",1);
auto b2 = create(1,"hello",5);
// [2] passing it directly to the ctor of B
B b3(1,"hello");
// fails with gcc when the noexcept doesn't use decay
// but only if "hello" is the last argument and passed via a function
auto b4 = create(1,"hello");
auto b5 = create("hello");
}
gcc的编译错误是:
<source>:13:40: error: invalid conversion from 'const char*' to 'char'
[-fpermissive]
13 | noexcept(noexcept(T{std::forward<U>(t)}))
| ~~~~~~~~~~~~~~~^~~
| |
| const char*
代码:https://godbolt.org/z/s7rf64
对这种奇怪行为有任何想法吗?这是gcc错误吗?还是确实需要std::decay
?
解决方法
在@Marek R的评论之后,这似乎是gcc中的错误。
以下情况对于gcc而不是对clang也无效:
template<typename T,typename U>
auto convert(U&& t) {
// fails on gcc:
return T{std::forward<U>(t)};
// works ok if it is not brace initialization:
// return T(std::forward<U>(t));
}
template<std::size_t INDEX,typename T,typename U,typename... Us>
auto convert(U&& t,Us&&... ts) {
if constexpr(INDEX == 0) {
return convert<U>(t);
// works well if we add decay
// return convert<std::decay_t<U>>(t);
}
else {
return convert<INDEX-1,T>(ts...);
}
}
int main() {
// fails with gcc
auto p = convert<1,const char*>(1,"hello");
}