c++: Implement C++26 P3144R2 - Deleting a Pointer to an Incomplete Type Should be...

c++: Implement C++26 P3144R2 - Deleting a Pointer to an Incomplete Type Should be Ill-formed [PR115747]

The following patch implements the C++26 paper which makes delete
and delete[] on incomplete class types invalid, previously it has
been UB unless the class had trivial destructor and no custom
deallocator.

The patch uses permerror_opt, so -Wno-delete-incomplete makes it
still compile without warnings like before, and -fpermissive makes
it warn but not error; in SFINAE contexts it is considered an error
in C++26 and later.

2024-07-02  Jakub Jelinek  <jakub@redhat.com>
	    Jason Merrill  <jason@redhat.com>

	PR c++/115747
gcc/cp/
	* init.cc: Implement C++26 P3144R2 - Deleting a Pointer to an
	Incomplete Type Should be Ill-formed.
	(build_vec_delete_1): Emit permerror_at and return error_mark_node
	for delete [] on incomplete type.
	(build_delete): Similarly for delete.
gcc/testsuite/
	* g++.dg/init/delete1.C: Adjust expected diagnostics for C++26.
	* g++.dg/warn/Wdelete-incomplete-1.C: Likewise.
	* g++.dg/warn/incomplete1.C: Likewise.
	* g++.dg/ipa/pr85607.C: Likewise.
	* g++.dg/cpp26/delete1.C: New test.
	* g++.dg/cpp26/delete2.C: New test.
	* g++.dg/cpp26/delete3.C: New test.