Defined in header
template< class F, class... Args >
concept regular_invocable = std::invocable<F, Args...>;
invocable concept specifies that a callable type
F can be called with a set of argument
Args... using the function template std::invoke.
regular_invocable concept adds to the
invocable concept by requiring the
invoke expression to be equality preserving and not modify either the function object or the arguments.
 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).
In specification of standard concepts, operands are defined as the largest subexpressions that include only:
The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.
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.
Unless noted otherwise, every expression used in a requires-expression is required to be equality preserving and stable, and the evaluation of the expression may only modify its non-constant operands. Operands that are constant must not be modified.
The distinction between
regular_invocable is purely semantic.