If the diagnostic is to be emitted only after the function
has been selected by overload resolution,
an implementation might express such a condition
via a constraint-expression
and also define the function as deleted.
Whenever the Effects element specifies that the semantics of some function
F are Equivalent to some code sequence, then the various elements are
interpreted as follows.
If F's semantics specifies any Constraints or Mandates elements,
then those requirements are logically imposed prior to the equivalent-to semantics.
Next, the semantics of the code sequence are determined by the
Constraints, Mandates, Preconditions, Effects,
Synchronization, Postconditions, Returns, Throws,
Complexity, Remarks, and Error conditions
specified for the function invocations contained in the code sequence.
The value returned from F is specified by F's Returns element,
or if F has no Returns element,
a non-void return from F is specified by the
return statements ([stmt.return]) in the code sequence.
If F's semantics contains a Throws,
Postconditions, or Complexity element,
then that supersedes any occurrences of that element in the code sequence.
For non-reserved replacement and handler functions,
[support] specifies two behaviors for the functions in question:
their required and default behavior.
The default behavior
describes a function definition provided by the implementation.
The required behavior
describes the semantics of a function definition provided by
either the implementation or a C++ program.
Where no distinction is explicitly made in the description, the
behavior described is the required behavior.
Complexity requirements specified in the library clauses are upper bounds,
and implementations that provide better complexity guarantees meet
the requirements.