[PATCH] PR libstdc++/80229 restore support for shared_ptr<function type>

[Jonathan Wakely <jwakely at redhat dot com>]

My recent refactoring of enable_shared_from_this broke support for an
obscure use case, where shared_ptr "owns" a pointer to function (and
the deleter does something with it, like unregister a callback from a
plugin).

The problem is simply that I made the new
_M_enable_shared_from_this_with overloads take a const _Yp* parameter,
so that _Yp would be deduced as the non-const type. That was just
laziness, to save using remove_const on the type. But that breaks
function pointers, because const _Yp* won't deduce a type like
void(*)(), and so there is no viable overload of
_M_enable_shared_from_this_with.

The fix is just to use a parameter of _Yp* and then use remove_cv<_Yp>
to get the unqualified type.

I'm also adjusting a test which uses const_pointer_cast<X> to cast a
shared_ptr<const X> to shared_ptr<X>, but the object it owns was born
const so casting it away and accessing it as non-const is undefined.

	PR libstdc++/80229
	* include/bits/shared_ptr_base.h
	(__shared_ptr::_M_enable_shared_from_this_with): Change parameters to
	non-const and then use remove_cv to get unqualified type.
	* testsuite/20_util/enable_shared_from_this/members/const.cc: Don't
	cast away constness on object created const.
	* testsuite/20_util/shared_ptr/cons/80229.cc: New test.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document