### 12.3.2 Viable functions [over.match.viable]

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 parameter has a default argument ([dcl.fct.default]).127
For 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 ([temp.constr.decl]), those constraints shall be satisfied ([temp.constr.constr]).
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 parameter must also have default arguments.