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2767. not_fn call_wrapper can form invalid types

Section: 22.10.13 [func.not.fn] Status: C++17 Submitter: Jonathan Wakely Opened: 2016-08-19 Last modified: 2021-06-06

Priority: 0

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Discussion:

The definition of the call_wrapper type in the C++17 CD means this fails to compile:

#include <functional>

struct abc { virtual void f() const = 0; };
struct derived : abc { void f() const { } };
struct F { bool operator()(abc&) { return false; } };
derived d;
bool b = std::not_fn(F{})(static_cast<abc&&>(d));

The problem is that the return types use result_of_t<F(abc)> and F(abc) is not a valid function type, because it takes an abstract class by value.

The return types should use result_of_t<F(Args&&...)> instead.

[2016-09-09 Issues Resolution Telecon]

P0; move to Tentatively Ready

Proposed resolution:

This wording is relative to N4606.

1. Modify [func.not_fn], class call_wrapper synopsis, as indicated:

   class call_wrapper
   {
     […]
     template<class... Args>
       auto operator()(Args&&...) &
         -> decltype(!declval<result_of_t<FD&(Args&&...)>>());
     template<class... Args>
       auto operator()(Args&&...) const&
         -> decltype(!declval<result_of_t<FD const&(Args&&...)>>());
     template<class... Args>
       auto operator()(Args&&...) &&
         -> decltype(!declval<result_of_t<FD(Args&&...)>>());
     template<class... Args>
       auto operator()(Args&&...) const&&
         -> decltype(!declval<result_of_t<FD const(Args&&...)>>());
     […]
   };
   

2. Modify the prototype declarations of [func.not_fn] as indicated:

   template<class... Args>
     auto operator()(Args&&... args) &
       -> decltype(!declval<result_of_t<FD&(Args&&...)>>());
   template<class... Args>
     auto operator()(Args&&... args) const&
       -> decltype(!declval<result_of_t<FD const&(Args&&...)>>());
   

   […]

   template<class... Args>
     auto operator()(Args&&... args) &&
       -> decltype(!declval<result_of_t<FD(Args&&...)>>());
   template<class... Args>
     auto operator()(Args&&... args) const&&
       -> decltype(!declval<result_of_t<FD const(Args&&...)>>());