Before C++20, constraining a template with SFINAE required verbose std::enable_if_t boilerplate that resulted in 200-line compiler error cascades when constraints failed.

In C++20, concepts allow clean declarative constraints:

template<typename T>
concept Numeric = std::is_arithmetic_v<T>;

template<Numeric T>
T Add(T a, T b) {
    return a + b;
}

If someone passes std::string, Clang outputs: 'std::string' does not satisfy concept 'Numeric'. One clean, human-readable error line instead of a wall of template substitution failures!

Concepts also allow overload resolution based on constraint subsumption (more specific concept overloads beat general ones automatically).

Migrated our math engine from std::enable_if to concepts. Compilation times dropped by 18% because the compiler doesn't need to eagerly evaluate failed SFINAE template instantiations.