Template Argument Deduction which function which is no where used/declared/defined

Unable to deduce the meaning of the arguments provided in this template. Meaning of the scopeResolution operator with & before class-name and the to_fun which is no where defined/declared in the class.

Cam someone explain me this : class = decltype(&T::to_json) and to_json which has no where been declared/defined: Json(t.to_json()

template <class T, class = decltype(&T::to_json)>
    Json(const T & t) : Json(t.to_json()) {}