Possible regression in G++ 6.1.0

The following code

#include <string>
#include <map>
#include <cassert>

    struct       AV {
        explicit AV(std::string const&) {}
    };

#if 1
    static void check_cache_item(
        std::map<std::string, std::string> const& items) // FIXME remove
    {
        assert(!items.empty());
    }
#endif
    static void check_cache_item(
        std::map<std::string, AV> const& items)
    {
        assert(!items.empty());
    }


int main()
{
    check_cache_item({ { "id", "0" }, { "pk", "#0" } });
    check_cache_item({ { "id", "0" }, { "pk", "#1" } });
    check_cache_item({ { "id", AV{"0"} }, { "pk", AV{"#1"} } });
}

is accepted by g++ 4.8.4, g++ 5.3.0, clang++ 3.9.0; but g++ 6.1.0 gives an error:

cci.cc: In function ‘int main()’:
cci.cc:25:55: error: call of overloaded ‘check_cache_item(<brace-enclosed initializer list>)’ is ambiguous
     check_cache_item({ { "id", "0" }, { "pk", "#0" } });
                                                       ^
cci.cc:10:17: note: candidate: void check_cache_item(const std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >&)
     static void check_cache_item(
                 ^~~~~~~~~~~~~~~~
cci.cc:16:17: note: candidate: void check_cache_item(const std::map<std::__cxx11::basic_string<char>, AV>&)
     static void check_cache_item(
                 ^~~~~~~~~~~~~~~~
cci.cc:26:55: error: call of overloaded ‘check_cache_item(<brace-enclosed initializer list>)’ is ambiguous
     check_cache_item({ { "id", "0" }, { "pk", "#1" } });
                                                       ^
cci.cc:10:17: note: candidate: void check_cache_item(const std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >&)
     static void check_cache_item(
                 ^~~~~~~~~~~~~~~~
cci.cc:16:17: note: candidate: void check_cache_item(const std::map<std::__cxx11::basic_string<char>, AV>&)
     static void check_cache_item(
                 ^~~~~~~~~~~~~~~~
cci.cc: At global scope:
cci.cc:10:17: warning: ‘void check_cache_item(const std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >&)’ defined but not used [-Wunused-function]
     static void check_cache_item(
                 ^~~~~~~~~~~~~~~~

If I #ifdef out the first constructor, then every compiler throws an error (rightly, because the AV constructor is explicit).

Is this a regression in G++ 6.1.0 ?