CTAD and Deduction Guides
C++17 let the compiler infer template arguments from constructors; C++20 extended it to aliases. Deduction guides are how libraries steer that inference.
Class template argument deduction (CTAD)
std::pair p(1, 2.0); — no <int, double> needed; the constructor's parameters drive deduction. Without CTAD you would write std::pair<int, double> p(1, 2.0);.
Where inference goes wrong: iterator pairs
A constructor template <class It> Box(It first, It last) : items(first, last) {} would deduce Box<int*> from Box b(begin(arr), end(arr)); — the iterator type, not the element type. Deduction guides fix the inference without touching the class:
template <class It>
Box(It first, It last) -> Box<std::iter_value_t<It>>;
The guide says: "when you see this constructor signature, deduce from the value type instead." The standard library ships dozens: std::vector v(begin, end) deduces the element type this way.
C++20: alias CTAD
template <class T> using VecOf = std::vector<T>; — then VecOf v{1, 2, 3}; deduces std::vector<int>. Aliases became first-class deduction citizens in C++20.
Review skill: when a container type comes out "wrong" from a constructor call, suspect the missing guide before suspecting your code.