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, 9 insertions, 8 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 56e8038..e6812ac 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -4965,14 +4965,15 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
   bool IsDependent = false;
   const char *PrevSpec = nullptr;
   unsigned DiagID;
-  Decl *TagDecl = Actions.ActOnTag(
-      getCurScope(), DeclSpec::TST_enum, TUK, StartLoc, SS, Name, NameLoc,
-      attrs, AS, DS.getModulePrivateSpecLoc(), TParams, Owned, IsDependent,
-      ScopedEnumKWLoc, IsScopedUsingClassTag, BaseType,
-      DSC == DeclSpecContext::DSC_type_specifier,
-      DSC == DeclSpecContext::DSC_template_param ||
-          DSC == DeclSpecContext::DSC_template_type_arg,
-      &SkipBody).get();
+  Decl *TagDecl =
+      Actions.ActOnTag(getCurScope(), DeclSpec::TST_enum, TUK, StartLoc, SS,
+                    Name, NameLoc, attrs, AS, DS.getModulePrivateSpecLoc(),
+                    TParams, Owned, IsDependent, ScopedEnumKWLoc,
+                    IsScopedUsingClassTag,
+                    BaseType, DSC == DeclSpecContext::DSC_type_specifier,
+                    DSC == DeclSpecContext::DSC_template_param ||
+                        DSC == DeclSpecContext::DSC_template_type_arg,
+                    OffsetOfState, &SkipBody).get();
 
   if (SkipBody.ShouldSkip) {
     assert(TUK == Sema::TUK_Definition && "can only skip a definition");