c – std :: declval()触发断言错误,并在GCC中显示警告

#include <utility>

template <typename U>
auto foo() -> decltype(std::declval<U>() + std::declval<U>());

template <typename T>
decltype(foo<T>()) bar(T)
{}

int main()
{
    bar(1);
    return 0;
}

在使用-Wall -Wextra编译时,我在GCC的所有版本中都会发出警告和静态断言失败(4.7.3,4.8.1,4.9-some-git).例如,这是4.8.1的输出：

main.cpp: In instantiation of ‘decltype (foo<T>()) bar(T) [with T = int; decltype (foo<T>()) = int]’:
main.cpp:12:7:   required from here
main.cpp:8:2: warning: no return statement in function returning non-void [-Wreturn-type]
 {}
  ^
In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.1/include/g++-v4/bits/move.h:57:0,from /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.1/include/g++-v4/bits/stl_pair.h:59,from /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.1/include/g++-v4/utility:70,from main.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.1/include/g++-v4/type_traits: In instantiation of ‘typename std::add_rvalue_reference< <template-parameter-1-1> >::type std::declval() [with _Tp = int; typename std::add_rvalue_reference< <template-parameter-1-1> >::type = int&&]’:
main.cpp:8:2:   required from ‘decltype (foo<T>()) bar(T) [with T = int; decltype (foo<T>()) = int]’
main.cpp:12:7:   required from here
/usr/lib/gcc/x86_64-pc-linux-gnu/4.8.1/include/g++-v4/type_traits:1871:7: error: static assertion failed: declval() must not be used!
       static_assert(__declval_protector::__stop,

如果要么使用return语句禁用警告或供应栏,例如,

template <typename T>
decltype(foo<T>()) bar(T a)
{
    return a + a;
}

断言失败消失了.在任何情况下,Clang 3.3都不会触发断言错误.这是GCC的符合标准的行为吗？