[PATCH] Prevent __uses_alloc from holding dangling references

[Jonathan Wakely <jwakely at redhat dot com>]

The polymorphic_allocator::construct functions create dangling
pointers to rvalues of type memory_resource* and then dereference
them, leading to undefined behaviour.

This fixes those functions to use lvalues, and then adds a deleted
overload of __use_alloc to prevent this happening again.

This means __use_alloc can't be used like:

 f( __use_alloc<T, Alloc>(c.get_allocator()) );

because get_allocator() returns an rvalue, but it's an internal-only
helper and we can just do this instead:

 auto alloc = c.get_allocator();
 f( __use_alloc<T, Alloc>(alloc) );
 

	* include/bits/uses_allocator.h (__use_alloc(const _Alloc&&)): Add
	deleted overload to prevent dangling references to rvalues.
	* include/experimental/memory_resource
	(polymorphic_allocator::construct): Do not call __use_alloc with
	rvalue arguments.

Tested powerpc64le-linux, committed to trunk.

Attachment: patch.txt
Description: Text document