C++ Candidate template ignored: deduced conflicting types for parameter 'T' ('std::__1::basic_string<char>' vs. 'char [4]')

Question

I have the following code that fails when I call a bfs function and pass string literal as an argument:

template<typename T>
using Graph = unordered_map<T, vector<T> >;

template<typename T>
vector<T> bfs(const Graph<T>& graph, const T& root) {
    ...
}

int main() {
    Graph<string> social_network = {
        {"you", {"tony", "steve", "nick"}},
        {"tony", {"clint"}},
        {"nick", {"thor", "natasha"}},
        {"steve", {"phil", "clint"}}
    };

    bfs(social_network, "you"); <-- the problem is here
}

The error is the following:

Candidate template ignored: deduced conflicting types for parameter 'T' ('std::__1::basic_string<char>' vs. 'char [4]')

if I explicitly specify string type for the argument, the code compiles successfully:

bfs(social_network, string("you"));

Is there a way to pass a string literal as an argument and make this code compile as well:

bfs(social_network, "you");

Thanks!

Answer 1

You can make type of root as being ignored in deduction of template parameters by using type_identity_t (required c++20), then T is taken from graph parameter:

const std::type_identity_t<T>& root

Demo