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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman updated https://github.com/llvm/llvm-project/pull/130980

>From d03203db378196d3b32021c52c72d3993168faf4 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Wed, 12 Mar 2025 11:34:27 -0400
Subject: [PATCH 1/2] [C2y] Claim conformance to WG14 N3363

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.
---
 clang/test/C/C2y/n3363.c | 40 ++++++++++++++++++++++++++++++++++++++++
 clang/www/c_status.html  |  2 +-
 2 files changed, 41 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/C/C2y/n3363.c

diff --git a/clang/test/C/C2y/n3363.c b/clang/test/C/C2y/n3363.c
new file mode 100644
index 0000000000000..de73fd443227e
--- /dev/null
+++ b/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 {{'va_list' argument to 'va_start' is required to be an lvalue}}
+  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());
+  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}}
+};
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index 4432e62b47520..6080ed060cfbb 100644
--- a/clang/www/c_status.html
+++ b/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>

>From 75a0e72d417983a543544d6511ab2406a0c14982 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Wed, 12 Mar 2025 12:50:07 -0400
Subject: [PATCH 2/2] Fix test

---
 clang/test/C/C2y/n3363.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang/test/C/C2y/n3363.c b/clang/test/C/C2y/n3363.c
index de73fd443227e..2ad0241a74c60 100644
--- a/clang/test/C/C2y/n3363.c
+++ b/clang/test/C/C2y/n3363.c
@@ -21,10 +21,10 @@
 // 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 {{'va_list' argument to 'va_start' is required to be an lvalue}}
+  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());
+  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 *')}}
 }