[PATCH] Prepare shared_ptr for array support

[Jonathan Wakely <jwakely at redhat dot com>]

This patch doesn't change very much, except for adding
std::reinterpret_pointer_cast, but it makes it very easy to add array
support to shared_ptr, as defined for the Library Fundamentals TS by
https://wg21.link/n3920 and in the process of being added to C++17 by
https://wg21.link/p0414r1

The main change is to consistently constrain all template constructors
and assignment operators. In std::shared_ptr we just forward the
checks to the std::__shared_ptr base class, so all the hard work is
done there. The base uses traits that can easily be extended to handle
array types without touching the uses of those traits (I have that
patch ready).

We also need to use shared_ptr<T>::element_type* in several places
where we use T* today, because when T is an array type they are not
the same thing. e.g. shared_ptr<int[]>::get() return int* not int[]*.

In practical terms this changes very little, just makes some invalid
code SFINAE-able instead of giving errors during instantiation, and
gives a nicer static assertion message for invalid deleters.

	* include/backward/auto_ptr.h (__shared_ptr(auto_ptr&&))
	(shared_ptr(auto_ptr&&)): Adjust template parameter lists.
	* include/bits/shared_ptr.h (__sp_compatible_with)
	(__sp_is_constructible): New helper traits for shared_ptr.
	(shared_ptr::_Convertible): Replace with _Constructible.
	(shared_ptr::_Constructible, shared_ptr::_Assignable): Forward checks
	to base class.
	(shared_ptr::shared_ptr, shared_ptr::operator=): Constrain template
	with _Constructible and _Assignable.
	(shared_ptr::shared_ptr(shared_ptr<_Tp1>, _Tp*)): Use element_type
	instead of _Tp.
	(operator<): Likewise.
	(operator>): Define in terms of operator<.
	(static_pointer_cast, const_pointer_cast, dynamic_pointer_cast): Use
	element_type instead of _Tp.
	(reinterpret_pointer_cast): Define for C++17.
	(weak_ptr::_Convertible): Replace with _Constructible.
	(weak_ptr::_Constructible, weak_ptr::_Assignable): Forward checks
	to base class.
	(weak_ptr::weak_ptr, weak_ptr::operator=): Constrain templates
	with _Constructible and _Assignable.
	* include/bits/shared_ptr_base.h (__shared_ptr::_Convertible): Replace
	with _Compatible.
	(__shared_ptr::_SafeConv): New constraint for incoming raw pointers.
	(__shared_ptr::_Compatible): New constraint for converting from
	other types of shared_ptr and weak_ptr.
	(__shared_ptr::_Assignable): Define in terms of _Compatible.
	(__shared_ptr::_UniqCompatible, __shared_ptr::_UniqAssignable): New
	constraints for converting from unique_ptr.
	(__shared_ptr::__shared_ptr, __shared_ptr::operator=): Constrain
	template with _SaveConf, _Compatible and _Assignable. Remove
	__glibcxx_function_requires concept checks. Add static assertion for
	deleter expression being well-formed.
	(__shared_ptr::__shared_ptr(__shared_ptr<_Tp1>, _Tp*))
	(__shared_ptr::operator*, __shared_ptr::operator->)
	(__shared_ptr::get, __shared_ptr::_M_ptr): Use element_type instead
	of _Tp.
	(operator<): Likewise.
	(operator>): Define in terms of operator<.
	(static_pointer_cast, const_pointer_cast, dynamic_pointer_cast): Use
	element_type instead of _Tp.
	(reinterpret_pointer_cast): Define for C++17.
	(weak_ptr::_Convertible): Replace with _Compatible.
	(weak_ptr::_Compatible, weak_ptr::_Assignable): New constraints for
	conversions from other types of weak_ptr and shared_ptr.
	(__weak_ptr::__weak_ptr, __weak_ptr::operator=): Constrain templates
	with _Constructible and _Assignable.
	(__weak_ptr::_M_ptr): Use element_type instead of _Tp.
	* testsuite/20_util/shared_ptr/assign/auto_ptr_neg.cc: Adjust
	dg-error pattern.
	* testsuite/20_util/shared_ptr/cons/auto_ptr.cc: Test conversions.
	* testsuite/20_util/shared_ptr/cons/unique_ptr.cc: Likewise.
	* testsuite/20_util/shared_ptr/cons/void_neg.cc: Likewise.
	* testsuite/20_util/shared_ptr/casts/reinterpret.cc: New test.

Tested powerpc64le-linux and x86_64-linux, and also tested with the
libc++ and (parts of) the boost::shared_ptr tests.

Committed to trunk.

Attachment: patch.txt
Description: Text document