c++: reject scalar array initialization with nullptr [PR94510]

c++: reject scalar array initialization with nullptr [PR94510]

The change committed to GCC 9 to allow string literals as template arguments
caused the compiler to prune away, and thus miss diagnosing, conversion from
nullptr to int in an array initializer.  After looking at various approaches
to improving the pruning, we realized that the only place the pruning is
necessary is in the mangler.

gcc/cp/ChangeLog
2020-04-22  Martin Sebor  <msebor@redhat.com>
	    Jason Merrill  <jason@redhat.com>

	PR c++/94510
	* decl.c (reshape_init_array_1): Avoid stripping redundant trailing
	zero initializers...
	* mangle.c (write_expression): ...and handle them here even for
	pointers to members by calling zero_init_expr_p.
	* cp-tree.h (zero_init_expr_p): Declare.
	* tree.c (zero_init_expr_p): Define.
	(type_initializer_zero_p): Remove.
	* pt.c (tparm_obj_values): New hash_map.
	(get_template_parm_object): Store to it.
	(tparm_object_argument): New.

gcc/testsuite/ChangeLog
2020-04-22  Martin Sebor  <msebor@redhat.com>

	PR c++/94510
	* g++.dg/init/array58.C: New test.
	* g++.dg/init/array59.C: New test.
	* g++.dg/cpp2a/nontype-class34.C: New test.
	* g++.dg/cpp2a/nontype-class35.C: New test.