Diagnose extensions in 'offsetof'

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm made very
clear that it is an UB having type definitions with in offsetof.
Clang supports defining a type as the first argument as a conforming
extension due to how many projects use the construct in C99 and earlier
to calculate the alignment of a type. GCC also supports defining a type
as the first argument.

This adds extension warnings and documentation for the functionality
Clang explicitly supports.

Fixes #57065
Reverts the revert of 39da55e8f548a11f7dadefa73ea73d809a5f1729

Co-authored-by: Yingchi Long <i@lyc.dev>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

Differential Revision: https://reviews.llvm.org/D133574

1 files changed, 16 insertions, 13 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index af20c707..c94b7ec 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -16583,17 +16583,16 @@ static bool isAcceptableTagRedeclContext(Sema &S, DeclContext *OldDC,
 ///
 /// \param SkipBody If non-null, will be set to indicate if the caller should
 /// skip the definition of this tag and treat it as if it were a declaration.
-DeclResult Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
-                          SourceLocation KWLoc, CXXScopeSpec &SS,
-                          IdentifierInfo *Name, SourceLocation NameLoc,
-                          const ParsedAttributesView &Attrs, AccessSpecifier AS,
-                          SourceLocation ModulePrivateLoc,
-                          MultiTemplateParamsArg TemplateParameterLists,
-                          bool &OwnedDecl, bool &IsDependent,
-                          SourceLocation ScopedEnumKWLoc,
-                          bool ScopedEnumUsesClassTag,
-                          TypeResult UnderlyingType, bool IsTypeSpecifier,
-                          bool IsTemplateParamOrArg, SkipBodyInfo *SkipBody) {
+DeclResult
+Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
+               CXXScopeSpec &SS, IdentifierInfo *Name, SourceLocation NameLoc,
+               const ParsedAttributesView &Attrs, AccessSpecifier AS,
+               SourceLocation ModulePrivateLoc,
+               MultiTemplateParamsArg TemplateParameterLists, bool &OwnedDecl,
+               bool &IsDependent, SourceLocation ScopedEnumKWLoc,
+               bool ScopedEnumUsesClassTag, TypeResult UnderlyingType,
+               bool IsTypeSpecifier, bool IsTemplateParamOrArg,
+               OffsetOfKind OOK, SkipBodyInfo *SkipBody) {
   // If this is not a definition, it must have a name.
   IdentifierInfo *OrigName = Name;
   assert((Name != nullptr || TUK == TUK_Definition) &&
@@ -17366,10 +17365,14 @@ CreateNewDecl:
                                cast_or_null<RecordDecl>(PrevDecl));
   }
 
+  if (OOK != OOK_Outside && TUK == TUK_Definition && !getLangOpts().CPlusPlus)
+    Diag(New->getLocation(), diag::ext_type_defined_in_offsetof)
+        << (OOK == OOK_Macro) << New->getSourceRange();
+
   // C++11 [dcl.type]p3:
   //   A type-specifier-seq shall not define a class or enumeration [...].
-  if (getLangOpts().CPlusPlus && (IsTypeSpecifier || IsTemplateParamOrArg) &&
-      TUK == TUK_Definition) {
+  if (!Invalid && getLangOpts().CPlusPlus &&
+      (IsTypeSpecifier || IsTemplateParamOrArg) && TUK == TUK_Definition) {
     Diag(New->getLocation(), diag::err_type_defined_in_type_specifier)
       << Context.getTagDeclType(New);
     Invalid = true;