CWG Issue 300 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

300. References to functions in template argument deduction

Section: 13.10.3.6 [temp.deduct.type]Status: CD1Submitter: Andrei IltchenkoDate: 11 Jul 2001

[Moved to DR at October 2002 meeting.]

Paragraph 9 of 13.10.3.6 [temp.deduct.type] enumerates the forms that the types P and A need to have in order for template argument deduction to succeed.

For P denoting a pointer to function the paragraph lists the following forms as allowing for template argument deduction:

type()(T) T()() T(*)(T)

On the other hand, no provision has been made to accommodate similar cases for references to functions, which in light of the wording of 13.10.3.6 [temp.deduct.type] paragraph 11 means that the program below is ill-formed (some of the C++ compilers do not reject it however):

template<typename Arg, typename Result, typename T>
Result  foo_r(Result(& rf)(Arg), T x)
{   return  rf(Arg(x));   }

template<typename Arg, typename Result, typename T>
Result  foo_p(Result(* pf)(Arg), T x)
{   return  pf(Arg(x));   }

#include <iostream>
int  show_arg(char c)
{
   std::cout << c << ' ';
   if(std::cout)  return  0;
   return  -1;
}

int  main()
{
                                               // The deduction
   int  (& rf1)(int(&)(char), double) = foo_r; // shall fail here
                                               // While here
   int  (& rf2)(int(*)(char), double) = foo_p; // it shall succeed
   return  rf2(show_arg, 2);
}

Proposed resolution (10/01, same as suggested resolution):

In the list of allowable forms for the types P andA in paragraph 9 of 13.10.3.6 [temp.deduct.type] replace

type()(T) T()() T(*)(T)

by

type(T) T() T(T)