The type of inner struct.

inner_struct.cpp

#include <iostream>
#include <string_view>

template <class T>
constexpr std::string_view type_name(const T&)
{
    std::string_view sv = __PRETTY_FUNCTION__;

    sv.remove_prefix(sv.find('=') + 2);
    return { sv.data(), sv.find(';') };
}

auto test()
{
    struct inner { };
    return inner();
};

int main()
{   
    // Type of inner struct ...
    std::cout << type_name(test()) << '\n';

    // ... compared to type of lambda
    auto f = []{ };
    std::cout << type_name(f) << '\n';
}

Output

test()::inner
main()::<lambda()>