[C2y] Claim conformance to WG14 N3363 (#130980)

This paper clarifies two things:
  * a call to va_start must be within the body of a variadic function,
  * the va_list arguments must be lvalues

Clang has correctly diagnosed the first point since Clang 5.0. Clang
generally diagnoses the second point, except for va_copy. However, the
second point is not a constraint violation, so a diagnostic is not
strictly required. That said, it would be good for us to properly
diagnose va_copy.

Author: AaronBallman

clang/test/C/C2y/n3363.c

@@ -0,0 +1,40 @@
+// RUN: %clang_cc1 -triple x86_64-pc-linux -verify=expected,array-type -std=c2y -Wall -pedantic -ffreestanding %s
+// RUN: %clang_cc1 -triple aarch64-pc-win32 -verify=expected,char-ptr-type -std=c2y -Wall -pedantic -ffreestanding %s
+
+/* WG14 N3363: Clang 5
+ * stdarg.h wording... v3
+ *
+ * Clarifies that va_start can only be used within a function body, and that
+ * the macros and functions require an lvalue of type va_list instead of an
+ * rvalue.
+ *
+ * Clang has diagnosed this correctly in all cases except va_copy since
+ * Clang 5. va_copy still needs to be updated to issue a diagnostic about use
+ * of an rvalue. However, our lack of a diagnostic is still conforming because
+ * a diagnostic is not required (it's not a constraint violation).
+ */
+
+#include <stdarg.h>
+
+// The va_list argument given to every macro defined in this subclause shall be
+// an lvalue of this type or the result of array-to-pointer decay of such an
+// lvalue.
+void f(int a, ...) {
+  va_list rvalue(); // array-type-error {{function cannot return array type 'va_list' (aka '__builtin_va_list')}}
+  va_start(rvalue()); // / char-ptr-type-error {{non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')}}
+  va_arg(rvalue(), int); // char-ptr-type-error {{expression is not assignable}}
+  // FIXME: this should get two diagnostics about use of a non-lvalue.
+  va_copy(rvalue(), rvalue()); // char-ptr-type-error {{non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')}}
+  va_end(rvalue()); // char-ptr-type-error {{non-const lvalue reference to type '__builtin_va_list' cannot bind to a temporary of type 'va_list' (aka 'char *')}}
+}
+
+// The va_start macro may only be invoked in the compound-statement of the body
+// of a variadic function.
+void g(va_list ap, int [(va_start(ap), 1)], ...) { // expected-error {{'va_start' cannot be used outside a function}}
+  va_end(ap);
+}
+
+va_list ap;
+struct S {
+  int array[(va_start(ap), 1)]; // expected-error {{'va_start' cannot be used outside a function}}
+};

clang/www/c_status.html

@@ -263,7 +263,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>stdarg.h wording... v3</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3363.pdf">N3363</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="full" align="center">Clang 5</td>
 	</tr>
     <tr>
       <td>Preprocessor integer expressions, v. 2</td>