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 ([temp.constr.decl])
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, Expects, Effects,
Synchronization, Ensures, 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,
Ensures, or Complexity element,
then that supersedes any occurrences of that element in the code sequence.