[clang] [clang][CodeGen] Check initializer of zero-size fields for nullptr (PR #109271)

[Michael Buch via cfe-commits]

================
@@ -738,7 +738,7 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
     // Zero-sized fields are not emitted, but their initializers may still
     // prevent emission of this struct as a constant.
     if (isEmptyFieldForLayout(CGM.getContext(), Field)) {
-      if (Init->HasSideEffects(CGM.getContext()))
+      if (Init && Init->HasSideEffects(CGM.getContext()))
----------------
Michael137 wrote:

This is where C and C++ diverge (in `InitListChecker::PerformEmptyInit`):
```
  // C++ [dcl.init.aggr]p7:
  //   If there are fewer initializer-clauses in the list than there are
  //   members in the aggregate, then each member not explicitly initialized
  //   ...
  bool EmptyInitList = SemaRef.getLangOpts().CPlusPlus11 &&
      Entity.getType()->getBaseElementTypeUnsafe()->isRecordType();
  if (EmptyInitList) {
    // C++1y / DR1070:
    //   shall be initialized [...] from an empty initializer list.
    //
    // We apply the resolution of this DR to C++11 but not C++98, since C++98
    // does not have useful semantics for initialization from an init list.
    // We treat this as copy-initialization, because aggregate initialization
    // always performs copy-initialization on its elements.
    //
    // Only do this if we're initializing a class type, to avoid filling in
    // the initializer list where possible.
    InitExpr = VerifyOnly
                   ? &DummyInitList
                   : new (SemaRef.Context)
                         InitListExpr(SemaRef.Context, Loc, std::nullopt, Loc);
    InitExpr->setType(SemaRef.Context.VoidTy);
    SubInit = InitExpr;
    Kind = InitializationKind::CreateCopy(Loc, Loc);
  } else {
    // C++03:
    //   shall be value-initialized.
  }
```
Technically this was only allowed as a GNU extension in C until C23 AFAIU (and in C++ since C++11).

Should we allow this for `SemaRef.getLangOpts().C23` too perhaps?

@AaronBallman 

https://github.com/llvm/llvm-project/pull/109271