[Clang] [Sema] No longer diagnose type definitions in offsetof in C23 mode (#84169)

This is now allowed in C23; continue to diagnose it in earlier language
modes as before, but now as a C23 extension rather than a GNU extension.

This fixes #83658.

Author: Sirraide

clang/docs/ReleaseNotes.rst

@@ -307,6 +307,9 @@ Improvements to Clang's diagnostics
 - ``-Wmicrosoft``, ``-Wgnu``, or ``-pedantic`` is now required to diagnose C99
   flexible array members in a union or alone in a struct. Fixes GH#84565.
 
+- Clang now no longer diagnoses type definitions in ``offsetof`` in C23 mode.
+  Fixes #GH83658.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1748,8 +1748,8 @@ def err_type_defined_in_condition : Error<
 def err_type_defined_in_enum : Error<
   "%0 cannot be defined in an enumeration">;
 def ext_type_defined_in_offsetof : Extension<
-  "defining a type within '%select{__builtin_offsetof|offsetof}0' is a Clang "
-  "extension">, InGroup<GNUOffsetofExtensions>;
+  "defining a type within '%select{__builtin_offsetof|offsetof}0' is a C23 "
+  "extension">, InGroup<C23>;
 
 def note_pure_virtual_function : Note<
   "unimplemented pure virtual method %0 in %1">;

clang/lib/Sema/SemaDecl.cpp

@@ -18170,7 +18170,9 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
                                cast_or_null<RecordDecl>(PrevDecl));
   }
 
-  if (OOK != OOK_Outside && TUK == TUK_Definition && !getLangOpts().CPlusPlus)
+  // Only C23 and later allow defining new types in 'offsetof()'.
+  if (OOK != OOK_Outside && TUK == TUK_Definition && !getLangOpts().CPlusPlus &&
+      !getLangOpts().C23)
     Diag(New->getLocation(), diag::ext_type_defined_in_offsetof)
         << (OOK == OOK_Macro) << New->getSourceRange();
 

clang/test/C/C2x/n2350.c

@@ -5,18 +5,18 @@
 // RUN: %clang_cc1 -fsyntax-only -pedantic -Wno-comment -std=c99 -verify %s
 // RUN: %clang_cc1 -fsyntax-only -pedantic -Wno-comment -std=c11 -verify %s
 // RUN: %clang_cc1 -fsyntax-only -pedantic -Wno-comment -std=c17 -verify %s
-// RUN: %clang_cc1 -fsyntax-only -pedantic -Wno-comment -std=c2x -verify %s
+// RUN: %clang_cc1 -fsyntax-only -pedantic -Wno-comment -std=c2x -verify=silent %s
 
 // silent-no-diagnostics
 
 // Reject definitions in __builtin_offsetof
 // https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
 int simple(void) {
   return __builtin_offsetof(struct A // cpp-error {{'A' cannot be defined in a type specifier}} \
-                                        expected-warning {{defining a type within '__builtin_offsetof' is a Clang extension}}
+                                        expected-warning {{defining a type within '__builtin_offsetof' is a C23 extension}}
   {
     int a;
-    struct B // expected-warning {{defining a type within '__builtin_offsetof' is a Clang extension}}
+    struct B // expected-warning {{defining a type within '__builtin_offsetof' is a C23 extension}}
     {
       int c;
       int d;
@@ -26,7 +26,7 @@ int simple(void) {
 
 int anonymous_struct(void) {
   return __builtin_offsetof(struct // cpp-error-re {{'(unnamed struct at {{.*}})' cannot be defined in a type specifier}} \
-                                      expected-warning {{defining a type within '__builtin_offsetof' is a Clang extension}}
+                                      expected-warning {{defining a type within '__builtin_offsetof' is a C23 extension}}
   {
     int a;
     int b;
@@ -47,7 +47,7 @@ int struct_in_second_param(void) {
 
 int macro(void) {
   return offsetof(struct A // cpp-error {{'A' cannot be defined in a type specifier}} \
-                              expected-warning 2 {{defining a type within 'offsetof' is a Clang extension}}
+                              expected-warning 2 {{defining a type within 'offsetof' is a C23 extension}}
   {
     int a;
     struct B // verifier seems to think the error is emitted by the macro

clang/test/C/drs/dr4xx.c

@@ -1,7 +1,7 @@
-/* RUN: %clang_cc1 -std=c89 -verify=expected,c89only -pedantic -Wno-c11-extensions %s
-   RUN: %clang_cc1 -std=c99 -verify=expected -pedantic -Wno-c11-extensions %s
-   RUN: %clang_cc1 -std=c11 -verify=expected -pedantic %s
-   RUN: %clang_cc1 -std=c17 -verify=expected -pedantic %s
+/* RUN: %clang_cc1 -std=c89 -verify=expected,c89only,pre-c23 -pedantic -Wno-c11-extensions %s
+   RUN: %clang_cc1 -std=c99 -verify=expected,pre-c23 -pedantic -Wno-c11-extensions %s
+   RUN: %clang_cc1 -std=c11 -verify=expected,pre-c23 -pedantic %s
+   RUN: %clang_cc1 -std=c17 -verify=expected,pre-c23 -pedantic %s
    RUN: %clang_cc1 -std=c2x -verify=expected -pedantic %s
  */
 
@@ -343,10 +343,13 @@ void dr496(void) {
                                              */
 
   /* The DR asked a question about whether defining a new type within offsetof
-   * is allowed. C2x N2350 made this explicitly undefined behavior, but GCC and
-   * Clang both support it as an extension.
+   * is allowed. C23 N2350 had made this explicitly undefined behavior, but this
+   * was later overturned when C23 DE-137 was accepted, making it well-formed.
+   *
+   * Additionally, GCC and Clang both support it as an extension in pre-C23
+   * mode.
    */
-   (void)__builtin_offsetof(struct S { int a; }, a); /* expected-warning{{defining a type within '__builtin_offsetof' is a Clang extension}} */
+   (void)__builtin_offsetof(struct S { int a; }, a); /* pre-c23-warning{{defining a type within '__builtin_offsetof' is a C23 extension}} */
 }
 
 /* WG14 DR499: yes