[C2y] Claim nonconformance to WG14 N3348 (#166966)

This paper allows use of * in a multidimensional array extent within a
_Generic selection association, as a wildcard for any array extent.

Clang does not currently support this feature, so this is just some
initial test coverage along with an update to the conformance site.

Author: AaronBallman

clang/test/C/C2y/n3348.c

@@ -0,0 +1,44 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic %s
+
+/* WG14 N3348: No
+ * Matching of Multi-Dimensional Arrays in Generic Selection Expressions
+ *
+ * This allows use of * in a _Generic association as a placeholder for any size
+ * value.
+ *
+ * FIXME: Clang doesn't yet implement this paper. When we do implement it, we
+ * should expose the functionality in earlier language modes (C89) for
+ * compatibility with GCC.
+ */
+
+void test(int n, int m) {
+  static_assert(1 == _Generic(int[3][2], int[3][*]: 1, int[2][*]: 0));  /* expected-error {{star modifier used outside of function prototype}}
+                                                                           expected-error {{array has incomplete element type 'int[]'}}
+                                                                         */
+  static_assert(1 == _Generic(int[3][2], int[*][2]: 1, int[*][3]: 0));  // expected-error {{star modifier used outside of function prototype}}
+  static_assert(1 == _Generic(int[3][n], int[3][*]: 1, int[2][*]: 0));  /* expected-error {{star modifier used outside of function prototype}}
+                                                                           expected-error {{array has incomplete element type 'int[]'}}
+                                                                         */
+  static_assert(1 == _Generic(int[n][m], int[*][*]: 1, char[*][*]: 0)); /* expected-error 2 {{star modifier used outside of function prototype}}
+                                                                           expected-error {{array has incomplete element type 'int[]'}}
+                                                                         */
+  static_assert(1 == _Generic(int(*)[2], int(*)[*]: 1));                // expected-error {{star modifier used outside of function prototype}}
+}
+
+void questionable() {
+  // GCC accepts this despite the * appearing outside of a generic association,
+  // but it's not clear whether that's intentionally supported or an oversight.
+  // It gives a warning about * being used outside of a declaration, but not
+  // with an associated warning group.
+  static_assert(1 == _Generic(int[*][*], int[2][100]: 1)); /* expected-error 2 {{star modifier used outside of function prototype}}
+                                                              expected-error {{array has incomplete element type 'int[]'}}
+                                                            */
+  // GCC claims this matches multiple associations, so the functionality seems
+  // like it may be intended to work?
+  (void)_Generic(int[*][*], /* expected-error 2 {{star modifier used outside of function prototype}}
+                               expected-error {{array has incomplete element type 'int[]'}}
+                             */
+    int[2][100]: 1,
+    int[3][1000]: 2,
+  );
+}

clang/www/c_status.html

@@ -324,7 +324,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Matching of Multi-Dimensional Arrays in Generic Selection Expressions</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3348.pdf">N3348</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="none" align="center">No</td>
 	</tr>
     <tr>
       <td>The __COUNTER__ predefined macro</td>