std::predicate

template< class F, class... Args >
concept predicate =
  std::regular_invocable<F, Args...> &&
  boolean-testable<std::invoke_result_t<F, Args...>>;

The concept std::predicate<F, Args...> specifies that F is a predicate that accepts arguments whose types and value categories are encoded by Args..., i.e., it can be invoked with these arguments to produce a boolean-testable result.

Note that regular_invocable requires the invocation to not modify either the callable object or the arguments and be equality-preserving.

Equality preservation

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).