c: Implement C2y N3481 constraint against lvalue conversion with incomplete type

C2y replaces undefined behavior for lvalue conversion of an lvalue
with incomplete, non-array type with a constraint violation.
Implement this in GCC, which means disallowing lvalue conversion of
qualified or atomic void for C2y.  (Unqualified, non-atomic void is
excluded from the definition of "lvalue".)

I'm not convinced that the resolution of C90 DR#106 (which said that
certain cases with qualified void were valid) was really justified
even based on the wording of C90; nevertheless, this patch takes the
conservative approach of only disallowing qualified void here for C2y.

The test for this change showed that attempting to access an _Atomic
void object produced an ICE-after-error attempting the atomic load
logic for such a type.  require_complete_type returning
error_mark_node prevents this ICE from occurring any more in C2y mode;
to avoid it in older modes, a check of COMPLETE_TYPE_P is added to
really_atomic_lvalue.  I didn't find any existing bug report in
Bugzilla for this issue.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c/
	* c-typeck.cc (really_atomic_lvalue): Return false for incomplete
	types.
	(convert_lvalue_to_rvalue): Call require_complete_type for
	qualified void for C2y.

gcc/testsuite/
	* gcc.dg/c11-atomic-6.c, gcc.dg/c23-incomplete-1.c,
	gcc.dg/c2y-incomplete-3.c: New tests.

0 files changed, 0 insertions, 0 deletions