c: Implement C2Y alignof on incomplete arrays

C2Y has adopted support for alignof applied to incomplete array types
(N3273).  Add this support to GCC.  As the relevant checks are in
c-family code that doesn't have access to functions such as
pedwarn_c23, this remains a hard error for older versions and isn't
handled by -Wc23-c2y-compat, although preferably it would work like
pedwarn_c23 (pedwarn-if-pedantic for older versions, warning with
-Wc23-c2y-compat in C2Y mode).

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

gcc/c-family/
	* c-common.cc (c_sizeof_or_alignof_type): Allow alignof on an
	incomplete array type for C2Y.

gcc/testsuite/
	* gcc.dg/c23-align-10.c, gcc.dg/c2y-align-1.c,
	gcc.dg/c2y-align-2.c: New tests.

4 files changed, 23 insertions, 1 deletions

diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
index 24335de..7d752ac 100644
--- a/gcc/c-family/c-common.cc
+++ b/gcc/c-family/c-common.cc
@@ -3972,7 +3972,9 @@ c_sizeof_or_alignof_type (location_t loc,
       value = size_one_node;
     }
   else if (!COMPLETE_TYPE_P (type)
-	   && (!c_dialect_cxx () || is_sizeof || type_code != ARRAY_TYPE))
+	   && ((!c_dialect_cxx () && !flag_isoc2y)
+	       || is_sizeof
+	       || type_code != ARRAY_TYPE))
     {
       if (complain)
 	error_at (loc, "invalid application of %qs to incomplete type %qT",
diff --git a/gcc/testsuite/gcc.dg/c23-align-10.c b/gcc/testsuite/gcc.dg/c23-align-10.c
new file mode 100644
index 0000000..bd6b9c2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c23-align-10.c
@@ -0,0 +1,6 @@
+/* Test C2Y alignof on an incomplete array type: not allowed in C23.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+
+int a = alignof(int[]); /* { dg-error "incomplete" } */
+int b = alignof(int[][1]); /* { dg-error "incomplete" } */
diff --git a/gcc/testsuite/gcc.dg/c2y-align-1.c b/gcc/testsuite/gcc.dg/c2y-align-1.c
new file mode 100644
index 0000000..3f9ab18
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2y-align-1.c
@@ -0,0 +1,6 @@
+/* Test C2Y alignof on an incomplete array type.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2y -pedantic-errors" } */
+
+int a = alignof(int[]);
+int b = alignof(int[][1]);
diff --git a/gcc/testsuite/gcc.dg/c2y-align-2.c b/gcc/testsuite/gcc.dg/c2y-align-2.c
new file mode 100644
index 0000000..b7b8715
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2y-align-2.c
@@ -0,0 +1,8 @@
+/* Test C2Y alignof on an incomplete array type: still not allowed for other
+   incomplete types.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2y -pedantic-errors" } */
+
+int a = alignof(void); /* { dg-error "void" } */
+struct s;
+int b = alignof(struct s); /* { dg-error "incomplete" } */