From the set of candidate functions constructed for a given
context ([over.match.funcs]), a set of viable functions is
chosen, from which the best function will be selected by
comparing argument conversion sequences
and associated constraints ([temp.constr.decl])
for the best fit ([over.match.best]).

The selection of viable functions considers
associated constraints, if any, and
relationships between arguments and function parameters other
than the ranking of conversion sequences.

First, to be a viable function, a candidate function shall have
enough parameters to agree in number with the arguments in the
list.

- If there are m arguments in the list, all candidate functions having exactly m parameters are viable.
- A candidate function having fewer than m parameters is viable only if it has an ellipsis in its parameter list ([dcl.fct]).For the purposes of overload resolution, any argument for which there is no corresponding parameter is considered to “match the ellipsis” ([over.ics.ellipsis]) .
- A candidate function having more than m parameters is viable only if the (m+1)-st parameter has a default argument.131For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters.

Second, for a function to be viable, if it has associated constraints,
those constraints shall be satisfied ([temp.constr.decl]).

Third, for
F
to be a viable function, there shall exist for each
argument an
implicit conversion sequence that
converts that argument to the corresponding parameter of
F.

If the parameter has reference type, the implicit conversion sequence
includes the operation of binding the reference, and the fact that
an lvalue reference to non-const cannot be bound to an rvalue
and that an rvalue reference cannot be bound to an lvalue
can affect
the viability of the function (see [over.ics.ref]).

According to [dcl.fct.default],
parameters following the
(m+1)-st
parameter must also have default arguments.