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

[Aaron Ballman via cfe-commits]

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@@ -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()))
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AaronBallman wrote:

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

Empty initialization ends up in C's "default initialization" rules, which does:

> If an object that has automatic storage duration is not initialized explicitly, its representation is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, or any object is initialized with an empty initializer, then it is subject to default initialization, which initializes an object as follows:
— if it has pointer type, it is initialized to a null pointer;
— if it has decimal floating type, it is initialized to positive zero, and the quantum exponent is
implementation-defined;
— if it has arithmetic type, and it does not have decimal floating type, it is initialized to (positive
or unsigned) zero;
— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;
— if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits.

So yeah, I think we should do this in C23 mode.

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