[PATCH] PR libstdc++/87520 Always pass type-punned type_info reference

[Jonathan Wakely <jwakely at redhat dot com>]

The implementations of std::make_shared for -frtti and -fno-rtti are not
compatible, because they pass different arguments to
_Sp_counted_ptr_inplace::_M_get_deleter and so can't interoperate.
Either the argument doesn't match the expected value, and so the
shared_ptr::_M_ptr member is never set, or the type-punned reference is
treated as a real std::type_info object and gets dereferenced.

This patch removes the differences between -frtti and -fno-rtti, so that
typeid is never used, and the type-punned reference is used in both
cases. For backwards compatibility with existing code that passes
typeid(_Sp_make_shared_tag) that still needs to be handled, but only
after checking that the argument is not the type-punned reference (so
it's safe to treat as a real std::type_info object). The reference is
bound to an object of literal type, so that it doesn't need a guard
variable to make its initialization thread-safe.

This patch also fixes 87520 by ensuring that the type-punned reference
is bound to "a region of storage of suitable size and alignment to
contain an object of the reference's type" (as per the proposed
resolution of Core DR 453).

If all objects are built with the fixed version of GCC then -frtti and
-fno-rtti can be mixed freely and std::make_shared will work correctly.
If some objects are built with unfixed GCC versions then problems can
still arise, depending on which template instantiations are kept by the
linker.

	PR libstdc++/85930
	PR libstdc++/87520
	* include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_ti)
	[__cpp_rtti]: Define even when RTTI is enabled. Use array of
	sizeof(type_info) so that type-punned reference binds to an object
	of the correct size as well as correct alignment.
	(_Sp_counted_ptr_inplace::_M_get_deleter) [__cpp_rtti]: Check for
	_S_ti() reference even when RTTI is enabled.
	(__shared_ptr(_Sp_make_shared_tag, const _Alloc&, _Args&&...))
	[__cpp_rtti]: Pass _S_ti() instead of typeid(_Sp_make_shared_tag).

Tested x86_64-linux, committed to trunk. Backport to gcc-8-branch will
follow soon.

Attachment: patch.txt
Description: Text document