我有一个非常奇怪的编译问题与这个片段：

#include <any>
#include <gmock/gmock.h>

struct Class
{
    virtual std::any get(int, int) = 0;
};

struct MockClass: Class
{
    MOCK_METHOD2(get, std::any(int, int));
};

int foo()
{
    MockClass dd;
}

错误gcc 9.1.0：

/usr/include/c++/9.1.0/type_traits:131:12: error: incomplete type ‘std::is_copy_constructible<testing::internal::ReferenceOrValueWrapper<std::any> >’ used in nested name specifier

叮当声8.0.0：

/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/9.1.0/../../../../include/c++/9.1.0/type_traits:132:31: error: no member named 'value' in 'std::is_copy_constructible<testing::internal::ReferenceOrValueWrapper<std::any> >'

如果我用std::string或任何其他通用类型替换std::any，代码将编译。