[PATCH] PR77990 refactor unique_ptr to encapsulate tuple

[Jonathan Wakely]

We are incorrectly requiring unique_ptr deleters to be copyable here:

      explicit
      unique_ptr(pointer __p) noexcept
      : _M_t(__p, deleter_type())
      { }

We could just do:

      explicit
      unique_ptr(pointer __p) noexcept
      : _M_t()
      { std::get<0>(_M_t) = __p; }

But having to deal directly with the std::tuple inside unique_ptr has
been bothering me for some time. The tuple is used so we get the empty
base-class optimisation for the deleter, but that implementation
detail 

This patch refactors unique_ptr to put the std::tuple member into a
new type which provides named accessors for the tuple elements, so we
can stop using get<0> and get<1>. That new type can also provide a
single-argument constructor to fix the copyable requirement for
deleters. This also removes the code for deducing the pointer type
which is duplciated in unique_ptr and unique_ptr<T[], D>, and while in
the neighbourhood I changed it from old-school SFINAE using overloaded
functions to the new hotness with __void_t<>.

I intend to commit this to trunk, but on the branches I'll just fix
the constructor as shown above, as it's a smaller change.


	PR libstdc++/77990
	* include/bits/unique_ptr.h (__uniq_ptr_impl): New type to
	encapsulate implementation details.
	(unique_ptr::unique_ptr(_Up)): Don't copy deleter object.
	(unique_ptr::get, unique_ptr::get_deleter, unique_ptr::release):
	Call member functions of implementation object.
	(unique_ptr<T[], D>): Likewise.
	* python/libstdcxx/v6/printers.py (UniquePointerPrinter): Adjust for
	new implementation.
	* python/libstdcxx/v6/xmethods.py (UniquePtrGetWorker): Likewise.
	* testsuite/20_util/unique_ptr/assign/48635_neg.cc: Adjust dg-error
	lines.
	* testsuite/20_util/unique_ptr/assign/cv_qual.cc: Likewise.
	* testsuite/20_util/unique_ptr/cons/cv_qual.cc: Likewise.
	* testsuite/20_util/unique_ptr/cons/77990.cc: New test.

Tested powerpc64le-linux.

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