The Requires Clause

The requires clause can take four kinds of requirements:

• Simple requirements
• Nested requirements
• Compound requirements
• Type requirements

Simple requirements: expressions only checked for valid syntax

template <typename T>
concept TinyType = requires(T t) {
  sizeof(T) <= 4;
};

Nested requirements: Also checks if the expression is true

template <typename T>
concept TinyType = requires(T t) {
  sizeof(T) <= 4;
  requires sizeof(T) <= 4;
};

Compound requirements: Checks for valid syntax, doesn't throw exceptions, and the result is convertible

#include <concepts>

template <typename T>
concept Addable = requires(T a, T b) {
  { a + b } noexcept -> std::convertible_to<int>;
};