[Bug libstdc++/54722] New: std::is_nothrow_default_constructible::value depends on whether destructor throws or not.

[cassio.neri at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54722



             Bug #: 54722

           Summary: std::is_nothrow_default_constructible<T>::value

                    depends on whether destructor throws or not.

    Classification: Unclassified

           Product: gcc

           Version: unknown

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: libstdc++

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: cassio.n...@gmail.com





Consider:



#include <iostream>

#include <type_traits>



struct foo {

  foo() noexcept {}

  ~foo() {}

};



int main() {

  std::cout << std::boolalpha;

  std::cout << std::is_nothrow_default_constructible<foo>::value << std::endl;

  return 0;

}



This should output 'true' but it outputs 'false'. Adding a 'noexcept'

specification to ~foo() makes the code to output the expected result.



Looking at the source, I guess, the reason lies on the implementation of this

helper class:



template<typename _Tp>

  struct __is_nt_default_constructible_atom

  : public integral_constant<bool, noexcept(_Tp())>

  { };



Indeed, the expression '_Tp()' if executed creates a temporary of type _Tp

whose lifetime ends immediately and ~Tp_ is called. Therefore,

'noexcept(_Tp())' is 'true' if and only if neither the constructor nor the

destructor throw.



I believe the below implementation fixes the problem (at least it does for the

example above):



template<typename _Tp>

  struct __is_nt_default_constructible_atom

  : public std::integral_constant<bool, noexcept(new (std::nothrow) _Tp)>

  { };