[PATCH] Make std::enable_shared_from_this cope with ambiguity

[Jonathan Wakely <jwakely at redhat dot com>]

This patch does three things:

1. Refactor std::enable_shared_from_this support code.

 Instead of several overloads of __enable_shared_from_this_helper
 that contain the same code but operating on slightly different types
 I've split it into two parts. Upcasting to an accessible+unambiguous
 enable_shared_from_this base class is done by new functions found by
 ADL, __enable_shared_from_this_base. That is called by a new
 template function __shared_ptr::_M_enable_shared_from_this_with()
 which actually does the enabling by calling _M_weak_assign.

 Calls to _M_enable_shared_from_this_with(p) closely match the
 wording from my https://wg21.link/p0033r1 paper ("enables
 shared_from_this with p") and are constrained so they don't give an
 error if the __enable_shared_from_this_base() call is ambiguous. We
 already gracefully handled ambiguous std::enable_shared_from_this
 bases (PR56383) but failed noisily if a type had a combination of
 std::enable_shared_from_this, std::__enable_shared_from_this and
 std::experimental::enable_shared_from_this bases. Now we treat those
 combinations gracefully.

2. Change std::experimental::enable_shared_from_this bases to be
 initialized independently of std::enable_shared_from_this bases.
 It's now possible to have both, and for both to be enabled. See
 enable_shared_from_this.cc which tests this.

 The standard says we have to enable shared_from_this for types with
 an accessible and unambiguous std::enable_shared_from_this base
 class, and we should be able to do that even if the class also has
 an experimental::enable_shared_from_this base class. Now we can.

 Potentially we could do the same for std::__enable_shared_from_this,
 but I'm happy for those bases to be considered ambiguous with
 std::enable_shared_from_this.

3. Don't enable shared_from_this for arrays stored in
 experimental::shared_ptr.  This matches the boost::shared_ptr
 semantics, and I intend to propose this as a fix for C++17 too. It
 doesn't make sense to treat the first element of an array specially
 and enable shared_from_this for the first element only.


	* include/backward/auto_ptr.h (__shared_ptr(auto_ptr&&)): Call
	_M_enable_shared_from_this_with instead of
	__enable_shared_from_this_helper.
	* include/bits/shared_ptr.h (__enable_shared_from_this_helper):
	Remove overload for std::enable_shared_from_this..
	(__enable_shared_from_this_base): Define friend function to select a
	std::enable_shared_from_this base class.
	* include/bits/shared_ptr_base.h (__enable_shared_from_this_helper):
	Remove all overloads.
	(__shared_ptr): Change all relevant constructors to call
	_M_enable_shared_from_this_with instead of
	__enable_shared_from_this_helper.
	(__shared_ptr::__efst_base_t, __shared_ptr::__has_efst_base): Helpers
	to detect accessible and unambiguous enable_shared_from_this bases.
	(__shared_ptr::_M_enable_shared_from_this_with): New function to
	replace __enable_shared_from_this_helper overloads.
	(__enable_shared_from_this_helper): Remove overload for
	std::__enable_shared_from_this.
	(__enable_shared_from_this_base): Define friend function to select a
	std::__enable_shared_from_this base class.
	* include/experimental/bits/shared_ptr.h (experimental::shared_ptr):
	Change relevant constructors to call _M_enable_shared_from_this_with.
	(experimental::shared_ptr::__efst_base_t)
	(experimental::shared_ptr::__has_efst_base): Helpers to detect
	accessible and unambiguous enable_shared_from_this bases.
	(experimental::shared_ptr::_M_enable_shared_from_this_with): Define.
	(experimental::__enable_shared_from_this_helper): Remove overload for
	std::experimental::enable_shared_from_this.
	(experimental::__expt_enable_shared_from_this_base): Define friend
	function to select a std::experimental::enable_shared_from_this base.
	* testsuite/experimental/memory/shared_ptr/cons/
	enable_shared_from_this.cc: New test.
	* testsuite/experimental/memory/shared_ptr/cons/unique_ptr_ctor.cc:
	Adjust expected behaviour for shared_ptr<A[]>.

Tested powerpc64le-linux, comitted to trunk.

Attachment: patch.txt
Description: Text document