c++: deleting explicitly-defaulted functions [PR116162]

This PR points out the we're not implementing [dcl.fct.def.default]
properly.  Consider e.g.

  struct C {
     C(const C&&) = default;
  };

where we wrongly emit an error, but the move ctor should be just =deleted.
According to [dcl.fct.def.default], if the type of the special member
function differs from the type of the corresponding special member function
that would have been implicitly declared in a way other than as allowed
by 2.1-4, the function is defined as deleted.  There's an exception for
assignment operators in which case the program is ill-formed.

clang++ has a warning for when we delete an explicitly-defaulted function
so this patch adds it too.

When the code is ill-formed, we emit an error in all modes.  Otherwise,
we emit a pedwarn in C++17 and a warning in C++20.

	PR c++/116162

gcc/c-family/ChangeLog:

	* c.opt (Wdefaulted-function-deleted): New.

gcc/cp/ChangeLog:

	* class.cc (check_bases_and_members): Don't set DECL_DELETED_FN here,
	leave it to defaulted_late_check.
	* cp-tree.h (maybe_delete_defaulted_fn): Declare.
	(defaulted_late_check): Add a tristate parameter.
	* method.cc (maybe_delete_defaulted_fn): New.
	(defaulted_late_check): Add a tristate parameter.  Call
	maybe_delete_defaulted_fn instead of giving an error.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wdefaulted-function-deleted.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/defaulted15.C: Add dg-warning/dg-error.
	* g++.dg/cpp0x/defaulted51.C: Likewise.
	* g++.dg/cpp0x/defaulted52.C: Likewise.
	* g++.dg/cpp0x/defaulted53.C: Likewise.
	* g++.dg/cpp0x/defaulted54.C: Likewise.
	* g++.dg/cpp0x/defaulted56.C: Likewise.
	* g++.dg/cpp0x/defaulted57.C: Likewise.
	* g++.dg/cpp0x/defaulted58.C: Likewise.
	* g++.dg/cpp0x/defaulted59.C: Likewise.
	* g++.dg/cpp0x/defaulted63.C: New test.
	* g++.dg/cpp0x/defaulted64.C: New test.
	* g++.dg/cpp0x/defaulted65.C: New test.
	* g++.dg/cpp0x/defaulted66.C: New test.
	* g++.dg/cpp0x/defaulted67.C: New test.
	* g++.dg/cpp0x/defaulted68.C: New test.
	* g++.dg/cpp0x/defaulted69.C: New test.
	* g++.dg/cpp23/defaulted1.C: New test.

0 files changed, 0 insertions, 0 deletions