Define as
the implicit conversion sequence that converts
the argument in the list to the type of
the parameter of viable function F.

[over.best.ics] defines the implicit conversion sequences and [over.ics.rank]
defines what it means for one implicit conversion sequence to be
a better conversion sequence or worse conversion sequence than
another.

Given these definitions,
a viable function is defined to be a
*better*
function than another viable function
if for all arguments i,
is not a worse conversion
sequence than , and then

- for some argument j, is a better conversion sequence than , or, if not that,
- the context is an initialization by user-defined conversion
(see [dcl.init],
[over.match.conv], and [over.match.ref])
and the standard conversion sequence
from the return type of to the destination type
(i.e., the type of the entity being initialized)
is a better conversion sequence than the standard conversion sequence
from the return type of to the destination type
or, if not that,[
*Example 1*: struct A { A(); operator int(); operator double(); } a; int i = a; // a.operator int() followed by no conversion is better than // a.operator double() followed by a conversion to int float x = a; // ambiguous: both possibilities require conversions, // and neither is better than the other —*end example*] - the context is an initialization by conversion function for direct
reference binding of a reference to function type, the
return type of F1 is the same kind of reference (lvalue or rvalue)
as the reference being initialized, and the return type of F2 is not
or, if not that,[
*Example 2*: template <class T> struct A { operator T&(); // #1 operator T&&(); // #2 }; typedef int Fn(); A<Fn> a; Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 —*end example*] - F1 is not a function template specialization and F2 is a function template specialization, or, if not that,
- F1 and F2 are function template specializations, and the function template for F1 is more specialized than the template for F2 according to the partial ordering rules described in [temp.func.order], or, if not that,
- F1 and F2 are non-template functions with the same parameter-type-lists, and F1 is more constrained than F2 according to the partial ordering of constraints described in [temp.constr.order], or if not that,
- F1 is a constructor for a class D,
F2 is a constructor for a base class B of D, and
for all arguments
the corresponding parameters of F1 and F2 have the same type
or, if not that,[
*Example 3*: struct A { A(int = 0); }; struct B: A { using A::A; B(); }; int main() { B b; // OK, B::B() } —*end example*] - F2 is a rewritten candidate ([over.match.oper]) and
F1 is not
or, if not that,[
*Example 4*: struct S { friend auto operator<=>(const S&, const S&) = default; // #1 friend bool operator<(const S&, const S&); // #2 }; bool b = S() < S(); // calls #2 —*end example*] - F1 and F2 are rewritten candidates, and
F2 is a synthesized candidate
with reversed order of parameters
and F1 is not
or, if not that[
*Example 5*: struct S { friend std::weak_ordering operator<=>(const S&, int); // #1 friend std::weak_ordering operator<=>(int, const S&); // #2 }; bool b = 1 < S(); // calls #2 —*end example*] - F1 and F2 are generated from class template argument deduction ([over.match.class.deduct]) for a class D, and F2 is generated from inheriting constructors from a base class of D while F1 is not, and for each explicit function argument, the corresponding parameters of F1 and F2 are either both ellipses or have the same type, or, if not that,
- F1 is generated from a
*deduction-guide*([over.match.class.deduct]) and F2 is not, or, if not that, - F1 is the copy deduction candidate and F2 is not, or, if not that,
- F1 is generated from a non-template constructor
and F2 is generated from a constructor template. [
*Example 6*: template <class T> struct A { using value_type = T; A(value_type); // #1 A(const A&); // #2 A(T, T, int); // #3 template<class U> A(int, T, U); // #4 // #5 is the copy deduction candidate, A(A) }; A x(1, 2, 3); // uses #3, generated from a non-template constructor template <class T> A(T) -> A<T>; // #6, less specialized than #5 A a(42); // uses #6 to deduce A<int> and #1 to initialize A b = a; // uses #5 to deduce A<int> and #2 to initialize template <class T> A(A<T>) -> A<A<T>>; // #7, as specialized as #5 A b2 = a; // uses #7 to deduce A<A<int>> and #1 to initialize —*end example*]

If there is exactly one viable function that is a better function
than all other viable functions, then it is the one selected by
overload resolution; otherwise the call is ill-formed.111

[*Example 7*: void Fcn(const int*, short);
void Fcn(int*, int);
int i;
short s = 0;
void f() {
Fcn(&i, s); // is ambiguous because &i → int* is better than &i → const int*
// but s → short is also better than s → int
Fcn(&i, 1L); // calls Fcn(int*, int), because &i → int* is better than &i → const int*
// and 1L → short and 1L → int are indistinguishable
Fcn(&i, 'c'); // calls Fcn(int*, int), because &i → int* is better than &i → const int*
// and 'c' → int is better than 'c' → short
}
— *end example*]

If the best viable function resolves to a function
for which multiple declarations were found, and
if any two of these declarations inhabit different scopes and
specify a default argument that made the function viable,
the program is ill-formed.

[*Example 8*: namespace A {
extern "C" void f(int = 5);
}
namespace B {
extern "C" void f(int = 5);
}
using A::f;
using B::f;
void use() {
f(3); // OK, default argument was not used for viability
f(); // error: found default argument twice
}
— *end example*]

111)111)

The algorithm
for selecting the best viable function is linear in the number
of viable
functions.

Run a simple tournament to find a function
W
that is not
worse than any
opponent it faced.

Although it is possible that another function
F
that
W
did not face
is at least as good as
W,
F
cannot be the best function because at some point in the
tournament
F
encountered another function
G
such that
F
was not better than
G.

Hence,
either W is
the best function or there is no best function.

So, make a second pass over
the viable
functions to verify that
W
is better than all other functions.