[C2y] Claim conformance to WG14 N3481

This paper made it a constraint violation to have an incomplete type
during lvalue conversion. Previously, this was undefined behavior.

Clang has always diagnosed this for non-void incomplete types. However,
Clang does allow derefencing a void pointer (with a diagnostic, so we
still meet the conformance requirements), but not for a value
computation.

Author: AaronBallman

clang/test/C/C2y/n3481.c

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -verify -std=c2y %s
+// RUN: %clang_cc1 -verify -std=c23 %s
+
+/* WG14 N3481: Yes
+ * Slay Some Earthly Demons XVI
+ *
+ * It was previously UB to use a non-array lvalue with an incomplete type in a
+ * context which required the value of the object. Clang has always diagnosed
+ * this as an error, except when the incomplete type is void. Then we allow the
+ * dereference, but not for a value computation.
+ */
+
+struct f *p;  // expected-note {{forward declaration of 'struct f'}}
+void g(void) {
+  (void)*p; // expected-error {{incomplete type 'struct f' where a complete type is required}}
+}
+
+void h(void *ptr) {
+  (void)*ptr; // expected-warning {{ISO C does not allow indirection on operand of type 'void *'}}
+  (*ptr)++;   /* expected-warning {{ISO C does not allow indirection on operand of type 'void *'}}
+                 expected-error {{cannot increment value of type 'void'}}
+               */
+}

clang/www/c_status.html

@@ -309,7 +309,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons XVI</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3481.pdf">N3481</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
 	</tr>
     <tr>
       <td>Slay Some Earthly Demons XVII</td>