std::function as template parameter

Question

I currently have a map, but for flexibility, I want to be able to assign a lambda expression, returning std::wstring as value

Answer 1

Both std::function and std::wstring have conversion operators that could take the literal wide string you are passing. In both cases the conversions are user defined and thus the conversion sequence takes the same precedence, causing the ambiguity. This is the root cause of the error.