std::function with noexcept in C++17

问题:

In C++17 noexcept has been added to the type system:

void r1( void (*f)() noexcept ) { f(); }
void foo() { throw 1; }

int main()
{
    r1(foo);
}

The latest versions of GCC and Clang in C++17 mode reject the call r1(foo), because void (*)() cannot be implicitly converted to void (*)() noexcept.

But with std::function instead:

#include <functional>

void r2( std::function<void() noexcept> f ) { f(); }
void foo() { throw 1; }

int main()
{
    r2(foo);
}

Clang accepts the program, apparently ignoring the noexcept specifier; and g++ gives a strange error regarding std::function<void() noexcept>.

What is the correct behaviour for this second program in C++17?

回答1:

std::function's definition hasn't changed in the current working draft:

template<class T>
class function; // not defined

template<class R, class... ArgTypes>
class function<R(ArgTypes...)> {
    /* ... */
};

Since void() noexcept doesn't match the partial specialization, std::function<void() noexcept> is an incomplete type. Both Clang and GCC trunk diagnose this accordingly.