Otherwise, at least recent Clang trunk with -std=gnu++17 complains

> In file included from lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0/functional:60:
> In file included from lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0/unordered_map:47:
> lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0/bits/unordered_map.h:1197:5: error: deduction guide template contains a template parameter that cannot be deduced
>     unordered_map(initializer_list<pair<_Key, _Tp>>, _Allocator)
>     ^
> lib/gcc/x86_64-pc-linux-gnu/8.0.0/../../../../include/c++/8.0.0/bits/unordered_map.h:1196:34: note: non-deducible template parameter (anonymous)
>            _RequireAllocator<_Allocator>>
>                                         ^
> 1 error generated.