Functions, overloads, templates, and concepts

Prototype: checked Int addition returning nullopt on overflow. Final: implement the constrained solve<T>(T,T) for every integral T except bool, returning optional<T> and nullopt when the mathematical sum is outside T range. Preserve the ArithmeticInteger constraint. No floating-point inputs or mixed-type overload are required. Validate before any overflowing signed arithmetic.

C++ contract conventions: numeric values use signed Int (long long); all inputs and intermediate arithmetic are within its representable range. Opt<T> means std::optional<T> and absence is std::nullopt; Vec<T> means std::vector<T>. Invalid documented arguments throw std::invalid_argument. Inputs passed by const reference remain caller owned. Returned containers and records own their contents. No runtime type coercion is required; C++ static types exclude many dynamic-language type errors. Unless a lesson states otherwise, string ordering is lexicographic over Unicode code points (Text) or UTF-8 identifiers (std::string).

template<class T> concept ArithmeticInteger = std::integral<T> && !std::same_as<T,bool>;
inline Opt<Int> prototype(Int a,Int b);
template<ArithmeticInteger T> Opt<T> solve(T a,T b);

Compile and run from this directory:

c++ -std=c++20 -O2 -Wall -Wextra -pedantic -pthread main.cpp -o practice
./practice --stage prototype
./practice --stage final

Edit exercise.hpp. Both stages start unfinished and must report failures until implemented. Keep main.cpp as the executable contract. The runner returns nonzero for a failed check, an unexpected exception, an invalid stage, or an empty suite. C++20 and only the standard library are required.