[llvm-bugs] [Bug 32979] New: private std::enable_shared_from_this prevents std::make_shared with C++1z

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=32979

            Bug ID: 32979
           Summary: private std::enable_shared_from_this prevents
                    std::make_shared with C++1z
           Product: libc++
           Version: 4.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: p_hampson at wargaming.net
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

As of C++17 (P0033R1 (#1) merged in March 2016 (#2)), `std::make_shared` (and
the shared_ptr constructors that 'enable shared_from_this') should only attempt
to use an "unambiguous and accessible" (#3) std::enable_shared_from_this.

Although this produces the odd behaviour that a private
std::enable_shared_from_this will not be initialised by std::make_shared, it is
certainly expected to compile.

The minimal code-sample that demonstrates this is below (#4)

#include <memory>
#include <iostream>
#include <cassert>

struct A : private std::enable_shared_from_this<A> {
    std::weak_ptr<A> get_weak() { return weak_from_this(); }
};

using APtr = std::shared_ptr<A>;
using AWPtr = std::weak_ptr<A>;

int main()
{
    APtr pA1 = std::make_shared<A>();
    AWPtr pWA2 = pA1->get_weak();
    APtr pA2 = pWA2.lock();
    std::cout << pA1.use_count() << "\n";
    std::cout << "shared_from_this " << (pWA2.expired() ? "not enabled" :
"enabled") << "\n";
    assert( pA1.use_count() == 1 );
    assert( pWA2.expired() );
}

The assertions pass as-of gcc 7.

#1 http://wg21.link/p0033r1
#2
https://github.com/cplusplus/draft/commit/4013f7279a3b6a2b39658bac034ac06b10db4fbd
#3 [util.smartptr.shared.const]/1
http://eel.is/c++draft/util.smartptr.shared.const
#4 https://wandbox.org/permlink/syLLIeqUxWprhUod

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