C++ :: Passing Lambda As Template Parameter Not Working Correctly

I've was trying out a function template to automatically get the type of a lambda, but it seems that it won't compile

I've tried two different ways:

1.
template<class HASHER>
auto make_unordered_map(size_t bucketCount, HASHER const && hf)
-> unordered_map<string const, HASHER>&& {
return unordered_map<string const, int, HASHER>(bucketCount, hf);
} auto x = make_unordered_map(1, [](string const& key)->size_t { return key[0]; });

2.
template<class HASHER>
auto make_unordered_map(size_t bucketCount, HASHER const && hf2)
-> unordered_map<string const, int, decltype(hf2)> {
return unordered_map<string const, int, decltype(hf2)>(bucketCount, hf2);
} auto x = make_unordered_map(1, [](string const& key)->size_t { return key[0]; });

The test code are located here:

1. [URL] ....
2. [URL] ....

They are both based on the code that is stated to work in those examples. I.e.:

auto hf = [](string const& key)->size_t { return key[0]; };
unordered_map<string const, int, decltype(hf)> m (1, hf);