[C2y] Add test coverage and documentation for WG14 N3341

This paper made empty structures and unions implementation-defined. We
have always supported this as a GNU extension, so now we're documenting
our behavior and removing the extension warning in C2y mode.

Author: AaronBallman

clang/docs/LanguageExtensions.rst

@@ -5974,3 +5974,12 @@ Clang guarantees the following behaviors:
   padding bits are initialized to zero.
 
 Currently, the above extension only applies to C source code, not C++.
+
+Empty Objects in C
+==================
+The declaration of a structure or union type which has no named members is
+undefined behavior (C23 and earlier) or implementation-defined behavior (C2y).
+Clang allows the declaration of a structure or union type with no named members
+in all C language modes. `sizeof` for such a type returns `0`, which is
+different behavior than in C++ (where the size of such an object is typically
+`1`).

clang/docs/ReleaseNotes.rst

@@ -298,6 +298,12 @@ C2y Feature Support
   paper adopts Clang's existing practice, so there were no changes to compiler
   behavior.
 
+- Implemented support for `N3341 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3341.pdf>`_
+  which makes empty structure and union objects implementation-defined in C.
+  ``-Wgnu-empty-struct`` will be emitted in C23 and earlier modes because the
+  behavior is a conforming GNU extension in those modes, but will no longer
+  have an effect in C2y mode.
+
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaDecl.cpp

@@ -19365,11 +19365,12 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
       }
 
       // Structs without named members are extension in C (C99 6.7.2.1p7),
-      // but are accepted by GCC.
-      if (NonBitFields == 0 && !getLangOpts().CPlusPlus) {
-        Diag(RecLoc, IsEmpty ? diag::ext_empty_struct_union :
-                               diag::ext_no_named_members_in_struct_union)
-          << Record->isUnion();
+      // but are accepted by GCC. In C2y, this became implementation-defined
+      // (C2y 6.7.3.2p10).
+      if (NonBitFields == 0 && !getLangOpts().CPlusPlus && !getLangOpts().C2y) {
+        Diag(RecLoc, IsEmpty ? diag::ext_empty_struct_union
+                             : diag::ext_no_named_members_in_struct_union)
+            << Record->isUnion();
       }
     }
   } else {

clang/test/C/C2y/n3341.c

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic %s
+// RUN: %clang_cc1 -verify=gnu -Wall -pedantic %s
+
+/* WG14 N3341: Yes
+ * Slay Some Earthly Demons III
+ *
+ * Empty structure and union objects are now implementation-defined.
+ */
+
+// expected-no-diagnostics
+
+struct R {};               // gnu-warning {{empty struct is a GNU extension}}
+struct S { struct { }; };  // gnu-warning {{empty struct is a GNU extension}}
+struct T { int : 0; };     // gnu-warning {{struct without named members is a GNU extension}}
+union U {};                // gnu-warning {{empty union is a GNU extension}}
+