[C2y] Claim conformance to WG14 N3525 (llvm#166824)

The paper is ensuring that a static_assert operand can not be deferred
until runtime; it must accept an integer constant expression which is
resolved at compile time.

Note, Clang extends what it considers to be a valid integer constant
expression, so this also verifies the expected extension diagnostics.

Author: AaronBallman

clang/test/C/C2y/n3525.c

@@ -0,0 +1,30 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic %s
+// RUN: %clang_cc1 -verify -std=c23 -Wall -pedantic %s
+
+/* WG14 N3525: Yes
+ * static_assert without UB
+ *
+ * Ensures that a static_assert declaration cannot defer to runtime; it must
+ * take an integer constant expression that is resolved at compile time.
+ *
+ * Note: implementations are free to extend what is a valid integer constant
+ * expression, and Clang (and GCC) does so. So this test is validating that
+ * we quietly accept a pasing assertion, loudly reject a failing assertion, and
+ * issue a pedantic diagnostic for the extension case.
+ */
+
+static_assert(1); // Okay
+
+static_assert(0); // expected-error {{static assertion failed}}
+
+extern int a;
+static_assert(1 || a); // expected-warning {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+
+static_assert(a);      // expected-error {{static assertion expression is not an integral constant expression}}
+static_assert(0 || a); // expected-error {{static assertion expression is not an integral constant expression}}
+
+// Note, there is no CodeGen test for this; we have existing tests for the ICE
+// extension, so the pedantic warning is sufficient to verify we're not
+// emitting code which reads 'a' in '1 || a' because of the folding, and
+// there's no way to generate code for reading 'a' in '0 || a' because of the
+// error.

clang/www/c_status.html

@@ -344,7 +344,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>static_assert without UB</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3525.htm">N3525</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Yes</td>
 	</tr>
     <tr>
       <td>Allow calling static inline within extern inline</td>