Here’s the snippet of code

#include<memory>
#include<unordered_map>

struct
__declspec(dllexport)
Foo {
    std::unordered_map<const int*, std::unique_ptr<int>> foo;
};

Foo foo();

which fails like this:

C:/data/msvc/14.39.33321-Pre/includelist(1299): error C2679: binary '=': no operator found which takes a right-hand operand of type 'const std::pair<const int *const ,std::unique_ptr<int,std::default_delete<int>>>' (or there is no acceptable conversion)
C:/data/msvc/14.39.33321-Pre/includeutility(315): note: could be 'std::pair<_Kty,_Ty> &std::pair<_Kty,_Ty>::operator =(volatile const std::pair<_Kty,_Ty> &)'
        with
        [
            _Kty=const int *,
            _Ty=std::unique_ptr<int,std::default_delete<int>>
        ]
C:/data/msvc/14.39.33321-Pre/includelist(1299): note: 'std::pair<_Kty,_Ty> &std::pair<_Kty,_Ty>::operator =(volatile const std::pair<_Kty,_Ty> &)': cannot convert argument 2 from 'const std::pair<const int *const ,std::unique_ptr<int,std::default_delete<int>>>' to 'volatile const std::pair<_Kty,_Ty> &'
        with
        [
            _Kty=const int *,
            _Ty=std::unique_ptr<int,std::default_delete<int>>
        ]
C:/data/msvc/14.39.33321-Pre/includelist(1299): note: Reason: cannot convert from 'const std::pair<const int *const ,std::unique_ptr<int,std::default_delete<int>>>' to 'volatile const std::pair<_Kty,_Ty>'
        with
        [
            _Kty=const int *,
            _Ty=std::unique_ptr<int,std::default_delete<int>>
        ]

... see linked example for complete error

In the codebase where this has been extracted from, __declspec(dllexport) is the expansion of a macro that expands to different values on other platforms, e.g. __attribute__ ((visibility("default"))) on Linux, in which case the code compiles just fine.