A test snippet like this compiles successfully in C++14 mode but not in C++11 mode:

#include <unordered_map>
enum MyEnum { A, B, C };
std::unordered_map<MyEnum, int> myMap;

In C++11 mode, it fails like this:

In file included from libcxx-unordered-map.cpp:1:
In file included from /home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/unordered_map:435:
In file included from /home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/__hash_table:15:
In file included from /home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/memory:668:
/home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/type_traits:1811:38: error: implicit instantiation of undefined template 'std::hash<MyEnum>'
    : public integral_constant<bool, __is_empty(_Tp)> {};
                                     ^
/home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/unordered_map:453:18: note: in instantiation of template class 'std::is_empty<std::hash<MyEnum>>' requested here
          bool = is_empty<_Hash>::value && !__libcpp_is_final<_Hash>::value>
                 ^
/home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/unordered_map:936:13: note: in instantiation of default argument for '__unordered_map_hasher<MyEnum, std::__hash_value_type<MyEnum, int>, std::hash<MyEnum>, std::equal_to<MyEnum>>' required here
    typedef __unordered_map_hasher<key_type, __value_type, hasher, key_equal> __hasher;
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libcxx-unordered-map.cpp:3:33: note: in instantiation of template class 'std::unordered_map<MyEnum, int>' requested here
std::unordered_map<MyEnum, int> myMap;
                                ^
/home/martin/clang-nightly/x86_64-w64-mingw32/include/c++/v1/__memory/shared_ptr.h:1688:50: note: template is declared here
template <class _Tp> struct _LIBCPP_TEMPLATE_VIS hash;
                                                 ^
1 error generated.


This causes build errors when building libaom since https://aomedia-review.googlesource.com/c/aom/+/136604.