deduction guides for std::function

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(C++17) (C++23)
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( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* )
( until C++17* ) ( until C++17* ) ( until C++17* ) ( until C++17* )
( until C++20* )
( until C++20* )
( until C++17* ) ( until C++17* )
( until C++17* ) ( until C++17* )

( until C++17* )
( until C++17* ) ( until C++17* ) ( until C++17* ) ( until C++17* )
( until C++20* )
( until C++20* )
std::function
Defined in header <functional>
template < class R, class ... ArgTypes >
function ( R ( * ) ( ArgTypes... ) ) - > function < R ( ArgTypes... ) > ;
(1) (since C++17)
template < class F >
function ( F ) - > function < /*see below*/ > ;
(2) (since C++17)
template < class F >
function ( F ) - > function < /*see below*/ > ;
(3) (since C++23)
template < class F >
function ( F ) - > function < /*see below*/ > ;
(4) (since C++23)
1) This deduction guide is provided for std::function to allow deduction from functions.
2) This overload participates in overload resolution only if & F :: operator ( ) is well-formed when treated as an unevaluated operand and decltype ( & F :: operator ( ) ) is of the form R ( G :: * ) ( A... ) (optionally cv-qualified, optionally noexcept, optionally lvalue reference qualified). The deduced type is std:: function < R ( A... ) > .
3) This overload participates in overload resolution only if & F :: operator ( ) is well-formed when treated as an unevaluated operand and F :: operator ( ) is an explicit object parameter function whose type is of form R ( G, A... ) or R ( G, A... ) noexcept . The deduced type is std:: function < R ( A... ) > .
4) This overload participates in overload resolution only if & F :: operator ( ) is well-formed when treated as an unevaluated operand and F :: operator ( ) is a static member function whose type is of form R ( A... ) or R ( A... ) noexcept . The deduced type is std:: function < R ( A... ) > .

Notes

These deduction guides do not allow deduction from a function with ellipsis parameter , and the ... in the types is always treated as a pack expansion .

The type deduced by these deduction guides may change in a later standard revision (in particular, this might happen if noexcept support is added to std::function in a later standard).

Example

Run this code 
#include <functional>
int func(double) { return 0; }
int main() {
  std::function f{func}; // guide #1 deduces function<int(double)>
  int i = 5;
  std::function g = [&](double) { return i; }; // guide #2 deduces function<int(double)>
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3238 C++17 behavior of (2) was unclear when
F :: operator ( ) is &&-qualified 		clarified to be excluded from overload resolution
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