[C2y] Add documentation to conform to WG14 N3262; NFC

clang/docs/LanguageExtensions.rst

@@ -3977,7 +3977,10 @@ standard library ``<stdarg.h>`` header:
 
 * ``__builtin_va_list``
 
-A predefined typedef for the target-specific ``va_list`` type.
+A predefined typedef for the target-specific ``va_list`` type. It is undefined
+behavior to use a byte-wise copy of this type produced by calling ``memcpy``,
+``memmove``, or similar. Valid explicit copies are only produced by calling
+``va_copy`` or ``__builtin_va_copy``.
 
 * ``void __builtin_va_start(__builtin_va_list list, <parameter-name>)``
 

clang/test/C/C2y/n3262.c

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic %s
+// expected-no-diagnostics
+
+/* WG14 N3262: Yes
+ * Usability of a byte-wise copy of va_list
+ *
+ * NB: Clang explicitly documents this as being undefined behavior. A
+ * diagnostic is produced for some targets but not for others for assignment or
+ * initialization, but no diagnostic is possible to produce for use with memcpy
+ * in the general case, nor with a manual bytewise copy via a for loop.
+ *
+ * Therefore, nothing is tested in this file; it serves as a reminder that we
+ * validated our documentation against the paper. See
+ * clang/docs/LanguageExtensions.rst for more details.
+ *
+ * FIXME: it would be nice to add ubsan support for recognizing when an invalid
+ * copy is made and diagnosing on copy (or on use of the copied va_list).
+ */
+
+int main() {}

clang/www/c_status.html

@@ -1290,7 +1290,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Usability of a byte-wise copy of va_list</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3262.pdf">N3262</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
     </tr>
     <tr>
       <td>alignof of an incomplete array type</td>