c++ - 是否需要在noexcept运算符中进行std::decay？

Question

以下代码要求在gcc中的std::decay中使用noexcept operator，但在clang中不使用。

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
对这种奇怪的行为有任何想法吗？它是gcc错误吗？还是确实需要 std::decay？

Answer 1

在@Marek R发表评论之后，这似乎是gcc中的错误。
A bug was opened on gcc。
以下情况对于gcc而非clang也失败:

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");
}

代码: https://godbolt.org/z/jqxbfj