c++ - ADL using std types fails to find operator -

the following code fails compile

namespace { using c = std::vector<std::string>; std::ostream& operator << (std::ostream& lhs, const c& rhs) {     lhs << 5;     return lhs; } } int main() {     a::c f;     std::cout << f;     return 0; } 

with error

error   c2679   binary '<<': no operator found takes right-hand operand of type 'a::c' (or there no acceptable conversion)    

obviously cant find << operator presumably due considering c class std namespace. there way ensure compiler finds operator or otherwise work around problem?

a::c type alias, , aliases transparent. don't "remember" came from. when argument-dependent lookup , figure out associated namespaces are, consider associated namespaces of types - not alias got there. can't add associated namespaces existing types. specific associated namespace of f (which of type std::vector<std::string>) std, doesn't have operator<< associated it. since there's no operator<< found using ordinary lookup, nor there 1 found using adl, call fails.

now, said can't add associated namespaces existing types. can of course create new types:

namespace {     struct c : std::vector<std::string> { }; } 

or:

namespace {     // template parameters considered associated namespaces     struct s : std::string { };     using c = std::vector<s>; }