std::Common

template <class T, class U>
concept Common =
  std::Same<std::common_type_t<T, U>, std::common_type_t<U, T>> &&
  std::ConvertibleTo<T, std::common_type_t<T, U>> &&
  std::ConvertibleTo<U, std::common_type_t<T, U>> &&
  std::CommonReference<
    std::add_lvalue_reference_t<const T>,
    std::add_lvalue_reference_t<const U>> &&
  std::CommonReference<
    std::add_lvalue_reference_t<std::common_type_t<T, U>>,
    std::common_reference_t<
      std::add_lvalue_reference_t<const T>,
      std::add_lvalue_reference_t<const U>>>;

The concept Common<T, U> specifies that two types T and U share a common type (as computed by std::common_type_t) to which both can be converted.

Common<T, U> is satisfied only if, given expressions t and u such that decltype((t)) is T and decltype((u)) is U, if t and u do not modify their operands,

• two evaluations of std::common_type_t<T, U>(t) yield equal result if and only if t is an equality-preserving expression; and
• two evaluations of std::common_type_t<T, U>(u) yield equal result if and only if u is an equality-preserving expression.

In other words, the conversion to the common type must not alter the equality-preservation property of the original expression.

Equality preservation

An expression is equality preserving if it results in equal outputs given equal inputs.

• The inputs to an expression consist of its operands.
• The outputs of an expression consist of its result and all operands modified by the expression (if any).

Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.

See also