[C2y] Claim partial conformance to WG14 N3244

[AaronBallman]

This paper had several changes within it, and Clang implements some of the changes, but not others. This updates the status accordingly.

[AaronBallman]

The paper in question can be found at: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3244.pdf

[llvmbot]

@llvm/pr-subscribers-clang

Author: Aaron Ballman (AaronBallman)

Changes

This paper had several changes within it, and Clang implements some of the changes, but not others. This updates the status accordingly.

---

Full diff: https://github.com/llvm/llvm-project/pull/98525.diff

2 Files Affected:

• (added) clang/test/C/C2y/n3244.c (+65)
• (modified) clang/www/c_status.html (+9-1)

diff --git a/clang/test/C/C2y/n3244.c b/clang/test/C/C2y/n3244.c
new file mode 100644
index 0000000000000..f03533f93340f
--- /dev/null
+++ b/clang/test/C/C2y/n3244.c
@@ -0,0 +1,65 @@
+// RUN: %clang_cc1 -std=c2y %s -verify -Wno-gnu-alignof-expression
+
+/* WG14 N3244: Partial
+ * Slay Some Earthly Demons I
+ *
+ * NB: the committee adopted:
+ *   Annex J Item 21 (including additional change) -- no, we lack explicit documentation
+ *   Annex J Item 56 -- yes
+ *   Annex J Item 57 Option 1 -- yes
+ *   Annex J Item 67 -- no
+ *   Annex J Item 69 (alternative wording for semantics) -- no
+ */
+
+void reg_array(void) {
+  // Decay of an array with the register storage class specifier has gone from
+  // explicit undefined behavior to be implementation defined instead. Clang
+  // does not support this.
+  register int array[10];
+  (void)sizeof(array); // okay
+  int *vp = array;    // expected-error {{address of register variable requested}}
+  int val = array[0]; // expected-error {{address of register variable requested}}
+}
+
+struct F;  // expected-note {{forward declaration of 'struct F'}}
+void incomplete_no_linkage(struct F); // okay
+void incomplete_no_linkage(struct F f) { // expected-error {{variable has incomplete type 'struct F'}}
+  struct G g; // expected-error {{variable has incomplete type 'struct G'}} \
+                 expected-note {{forward declaration of 'struct G'}}
+  int i[];    // expected-error {{definition of variable with array type needs an explicit size or an initializer}}
+}
+
+void block_scope_non_extern_func_decl(void) {
+  static void f(void); // expected-error {{function declared in block scope cannot have 'static' storage class}}
+  extern void g(void); // okay
+  __private_extern__ void h(void); // okay
+}
+
+// FIXME: this function should be diagnosed as it is never defined in the TU.
+extern inline void never_defined_extern_inline(void);
+
+// While this declaration is fine because the function is defined within the TU.
+extern inline void is_defined_extern_inline(void);
+extern inline void is_defined_extern_inline(void) {}
+
+int NoAlignmentOnOriginalDecl;
+// FIXME: the original declaration has no alignment specifier, so the
+// declaration below should be diagnosed due to the incompatible alignment
+// specifier.
+_Alignas(8) int NoAlignmentOnOriginalDecl;
+_Static_assert(_Alignof(NoAlignmentOnOriginalDecl) == 8, "");
+
+_Alignas(8) int AlignmentOnOriginalDecl;
+// FIXME: this should be accepted because the redeclaration has no alignment
+// specifier.
+int AlignmentOnOriginalDecl; // expected-error {{'_Alignas' must be specified on definition if it is specified on any declaration}}
+_Static_assert(_Alignof(AlignmentOnOriginalDecl) == 8, "");
+
+long long CompatibleAlignment;
+_Static_assert(_Alignof(CompatibleAlignment) == _Alignof(long long), "");
+_Alignas(_Alignof(long long)) long long CompatibleAlignment; // Okay, alignment is the same as the implied alignment
+
+_Alignas(_Alignof(long long)) long long CompatibleAlignment2;
+// FIXME: this should be accepted because the redeclaration has no alignment
+// specifier.
+long long CompatibleAlignment2; // expected-error {{'_Alignas' must be specified on definition if it is specified on any declaration}}
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index 890ac1cd92286..1b0cf4065019e 100644
--- a/clang/www/c_status.html
+++ b/clang/www/c_status.html
@@ -1265,7 +1265,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay some earthly demons I</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3244.pdf">N3244</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="partial" align="center">
       <!-- Voted in:
              Annex J Item 21 (including additional change)
              Annex J Item 56
@@ -1273,6 +1273,14 @@ <h2 id="c2y">C2y implementation status</h2>
              Annex J Item 67
              Annex J Item 69 (alternative wording for semantics)
       -->
+        <details><summary>Partial</summary>
+          Clang does not document the implementation-defined behavior for decay
+          of an array with the register storage class specifier. Clang does not
+          diagnose an <code>extern inline</code> function with no definition in
+          the TU. Clang accepts and rejects redeclarations with/without an
+          alignment specifier, depending on the order of the declarations.
+        </details>
+      </td>
     </tr>
     <tr>
       <td>Support ++ and -- on complex values</td>

[efriedma-quic]

LGTM

[mgorny]

The added test fails on 32-bit x86:

FAIL: Clang :: C/C2y/n3244.c (1461 of 20066)
******************** TEST 'Clang :: C/C2y/n3244.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 1: /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/x/y/clang-abi_x86_32.x86/bin/clang -cc1 -internal-isystem /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/x/y/clang-abi_x86_32.x86/bin/../../../../lib/clang/19/include -nostdsysteminc -std=c2y /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/clang/test/C/C2y/n3244.c -verify -Wno-gnu-alignof-expression
+ /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/x/y/clang-abi_x86_32.x86/bin/clang -cc1 -internal-isystem /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/x/y/clang-abi_x86_32.x86/bin/../../../../lib/clang/19/include -nostdsysteminc -std=c2y /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/clang/test/C/C2y/n3244.c -verify -Wno-gnu-alignof-expression
error: 'expected-error' diagnostics seen but not expected: 
  File /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/clang/test/C/C2y/n3244.c Line 59: static assertion failed due to requirement '_Alignof (CompatibleAlignment) == _Alignof(long long)': 
error: 'expected-note' diagnostics seen but not expected: 
  File /var/tmp/portage/sys-devel/clang-19.0.0_pre20240713/work/clang/test/C/C2y/n3244.c Line 59: expression evaluates to '8 == 4'
2 errors generated.

--

********************

[AaronBallman]

Thank you for letting me know, I've pushed a fix for that.

Curiously, it seems _Alignof on an expression always returns the preferred alignment and not the actual alignment. This is consistent with GCC, even if it's a bit surprising: https://godbolt.org/z/nvKx98ff9

[mgorny]

Thanks. I can confirm that the tests pass for me with the patch applied.