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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/102064] Wrong assignable check in uninitialized_fill
 and uninitialized_fill_n
Date: Wed, 25 Aug 2021 13:25:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102064

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The assignable checks are also missing for C++98, but there are types which=
 can
be constructed via std::uninitialized_foo that cannot be assigned to by
std::foo

#include <memory>
#include <algorithm>

struct X;

struct Y
{
  operator X() const;
};

struct X
{
private:
  void operator=3D(const Y&);
};

Y::operator X() const { return X(); }

int main()
{
  unsigned char buf[sizeof(X)];
  Y y;
  std::uninitialized_fill_n((X*)buf, 1, y);
}

This should copy-construct an X from the result of the conversion operator.=
 But
we optimize it to std::fill_n which tries to use the inaccessible assignmen=
t.

/usr/include/c++/11/bits/stl_algobase.h:912:18: error: =E2=80=98void X::ope=
rator=3D(const
Y&)=E2=80=99 is private within this context
  912 |         *__first =3D __value;
      |         ~~~~~~~~~^~~~~~~~~=