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template<typename T>
T add(T a, T b) {
    return a + b;
}

// error deduced conflicting types for parameter 'T'
// ('double' and 'int')
auto r = add(2.1, 5);

// works, but not very nice
auto r = add<double>(2.1, 5);

template<typename T>
T add(T a, T b) {
    return a + b;
}
// This to just work
auto r = add(2.1, 5);
auto epsilon = std::numeric_limits<double>::epsilon();
assert(std::abs(r - 7.1) < epsilon);

template<typename T>
T add(T a, std::????_??????????<T> b) {
    return a + b;
}

auto r = add(2.1, 5);
auto epsilon = std::numeric_limits<double>::epsilon();
assert(std::abs(r - 7.1) < epsilon);

template<typename T>
T add(T a, std::type_identity_t<T> b) {
    return a + b;
}

auto r = add(2.1, 5);
auto epsilon = std::numeric_limits<double>::epsilon();
assert(std::abs(r - 7.1) < epsilon);