Ret(&)(Args...)和Ret(Args...)&有什么区别？

Question

当我尝试像这样编写自己的decay_t时：

#include <utility>
template <class T>
struct auto_decay {
auto operator () () noexcept {
return std::declval<T>();
}
};
template <class T> using decay_t = decltype( (decl_as<decay_impl::auto_decay< T >>())() );

并使用以下命令进行测试：

#include <type_traits>
int main() {
static_assert(is_same<decay_t<int ()&>, int (*)()>{}());
}

我收到以下错误：

In file included from test_decay.cc:1:
In file included from ./../src/decay.hpp:4:
In file included from ./../src/utility.hpp:3:
./../src/add_reference.hpp:17:14: error: reference to function type 'int () &' cannot have '&' qualifier
typedef T&& type;
^
./../src/add_reference.hpp:20:1: note: in instantiation of template class 'nxwheels::concept_check::add_rvalue_reference<int () &>' requested here
using add_rvalue_reference_t = typename add_rvalue_reference<T>::type;
^
./../src/utility.hpp:14:1: note: in instantiation of template type alias 'add_rvalue_reference_t' requested here
auto declval() noexcept -> add_rvalue_reference_t<T>;
^
./../src/decay.hpp:10:16: note: while substituting explicitly-specified template arguments into function template 'declval' 
return declval<T>();
^
./../src/decay.hpp:16:46: note: in instantiation of member function 'nxwheels::concept_check::decay_impl::auto_decay<int () &>::operator()' requested here
template <class T> using decay_t = decltype( (decl_as<decay_impl::auto_decay< T >>())() );
^
test_decay.cc:17:17: note: in instantiation of template type alias 'decay_t' requested here
assert_same<decay_t<int (void)&>, int (*)(void)>();
^
In file included from test_decay.cc:1:
./../src/decay.hpp:10:16: error: call to function 'declval' that is neither visible in the template definition nor found by argument-dependent lookup
return declval<T>();
^
./../src/decay.hpp:16:46: note: in instantiation of member function 'nxwheels::concept_check::decay_impl::auto_decay<int () &>::operator()' requested here
template <class T> using decay_t = decltype( (decl_as<decay_impl::auto_decay< T >>())() );
^
test_decay.cc:17:17: note: in instantiation of template type alias 'decay_t' requested here
assert_same<decay_t<int (void)&>, int (*)(void)>();
^
./../src/utility.hpp:14:6: note: 'declval' should be declared prior to the call site
auto declval() noexcept -> add_rvalue_reference_t<T>;
^
In file included from test_decay.cc:2:
./../src/concepts/is_same.hpp:18:5: error: static_assert failed
static_assert(is_same_v<T1, T2>);
^             ~~~~~~~~~~~~~~~~~
test_decay.cc:17:5: note: in instantiation of function template specialization 'nxwheels::concept_check::assert_same<int, int (*)()>' requested here
assert_same<decay_t<int (void)&>, int (*)(void)>();
^
3 errors generated.

但是当我将测试更改为：

#include <type_traits>
int main() {
static_assert(is_same<decay_t<int (&)()>, int (*)()>{}());
}

然后测试过去了，我不明白int (&)()和int () &有什么区别.我检查了 std：：is_funcion 和声明函数，它们都说int () &，而不是int (&)()。

编辑：

顺便说一句，在 std：：is_funcion 的possible implementation部分，Ret (Args...) &是 std：：is_function 的专业化。他们会怎么做？

Answer 1

R(&)(Arg...)是对函数的引用， 而R(Arg...)&是声明的成员函数的表示法

struct test {
void func() &;
};

编辑：

#include <type_traits>
struct test {
void func1() &;
};
template<typename>
struct PM_traits {};
template<class T, class U>
struct PM_traits<U T::*> {
using member_type = U;
};
int main()
{
using P = PM_traits<decltype(&test::func1)>::member_type;
static_assert(std::is_function<P>::value);
}

这要归功于支持R(Arg...)&的std::is_function过载