Your first line printme({'a', 'b', 'c'}) is illegal because the template argument T could not be inferred. If you explicitly specify the template argument it will work, e.g. printme<vector<char>>({'a', 'b', 'c'}) or printme<initializer_list<char>>({'a', 'b', 'c'}).
The other ones you listed are legal because the argument has a well-defined type, so the template argument T can be deduced just fine.
Your snippet with auto also works because il is considered to be of type std::initializer_list<char>, and therefore the template argument to printme() can be deduced.
The only “funny” part here is that auto will pick the type std::initializer_list<char> but the template argument will not. This is because § 14.8.2.5/5 of the C++11 standard explicitly states that this is a non-deduced context for a template argument:
A function parameter for which the associated argument is an initializer list (8.5.4) but the parameter does not have std::initializer_list or reference to possibly cv-qualified std::initializer_list type. [Example:
template<class T> void g(T); g({1,2,3}); // error: no argument deduced for T— end example ]
However with auto, § 7.1.6.4/6 has explicit support for std::initializer_list<>
if the initializer is a braced-init-list (8.5.4), with
std::initializer_list<U>.